Modular Extensions London 2026: SIPs, Prefab & Off-Site Build Guide
Modular and off-site construction is no longer the preserve of commercial buildings or self-build plots. For London homeowners wanting a faster, less disruptive extension, it is increasingly the method of choice. Here is what it involves, what it costs, and what the planning and mortgage implications are.
Quick Answer
Modular extensions use the same planning rules as traditional builds — if you need planning permission for a traditional extension, you need it for a modular one too. SIPs and timber frame extensions typically cost £1,400–£2,200 per m² in London (inc VAT), compared to £1,800–£2,800/m² for traditional brick and block. The main advantages are speed (structural shell in days rather than months), reduced site disruption, and superior thermal performance.
£1,400–£2,200/m²
SIPs/modular cost
Days, not months
Shell erection time
U-value 0.18 W/m²K+
Thermal improvement
Check your specific property constraints
Free Property CheckWhat Is a Modular or Off-Site Extension?
Off-site construction means that significant parts of the extension are manufactured in a factory or workshop before being delivered and assembled on your property. This is in contrast to traditional brick and block construction, where everything is built in situ on your site from individual materials.
The term covers a spectrum of approaches — from panelised systems where structural panels are made off-site and assembled on your foundations, to fully volumetric modular pods that arrive as a complete room and are craned into position. What they all share is a shift of labour-intensive work from your garden into a controlled factory environment.
The main off-site construction types
SIPs — Structural Insulated Panels
The most widely used system for residential extensions. SIPs panels consist of a rigid foam insulation core (typically expanded polystyrene or polyisocyanurate) bonded between two structural boards — usually oriented strand board (OSB). Panels are cut to your architect's drawings in the factory and arrive ready to assemble. The structural shell can be erected in days. SIPs achieve excellent thermal performance — a 172mm panel typically achieves a U-value of 0.18 W/m²K or better, meeting 2026 building regulations for walls without additional insulation layers. Typical supply-only cost: £70–£100/m² for panels.
Timber frame panel systems
Factory-made structural timber frames with insulation, membranes, and sometimes external cladding pre-fitted. Delivered as flat panels and assembled on site. Faster than traditional masonry but not quite as fast as SIPs. Familiar to most builders, which means more competitive labour rates for assembly. Timber frame extensions cost £1,400–£2,000/m² in London inc VAT for a complete extension, somewhat less than the equivalent SIPs build.
Steel frame systems
Light gauge steel frame (LGSF) panels are manufactured off-site and assembled on your foundations. Steel frame offers high dimensional accuracy and is particularly well-suited to tight urban plots where precise tolerances matter. It is also useful for extensions requiring large spans — wide-open kitchen diners where columns would be unacceptable. Steel frame typically costs £1,600–£2,400/m² in London inc VAT, slightly above timber frame due to material cost.
Volumetric modular pods
The most off-site end of the spectrum. A self-contained three-dimensional module — complete with internal lining, services, and sometimes fixtures — is manufactured in the factory and delivered as a finished unit. Requires a crane for installation. Highly controlled quality and extremely fast on-site installation (sometimes a single day), but the least architecturally flexible option. Better suited to rear garden rooms, annexe pods, or studio spaces than complex multi-room extensions that integrate deeply into the existing house. Typical cost: £1,800–£2,800/m² in London inc VAT for a finished module.
Modular vs Traditional Build: The Real Differences
The marketing around modular construction tends to oversimplify. The speed and disruption benefits are genuine — but modular is not simply cheaper, and it is not always the right choice for every London property or project type. Here is an honest comparison.
Speed
This is where modular genuinely wins. A traditional single-storey rear extension in London typically has 8–16 weeks on site from breaking ground to plastered walls. A SIPs or timber frame extension of equivalent size can have its structural shell erected in two to five days. Total on-site programme to the same stage: typically three to six weeks. For a family living in a London terrace during a build, this difference is substantial.
The reason is that factory production and site preparation happen concurrently. While your foundations and oversite slab are being poured on site, your SIPs panels are being cut and assembled in the factory. They arrive ready to go up the day after the slab has cured.
Disruption
Reduced on-site programme means less dust, noise, and contractor presence in your garden. The factory-cut panels arrive and go up quickly; there is no prolonged masonry work, no plasterers on site for weeks, no extended period where the back of your house is open to the elements. For households with young children or anyone working from home, this matters.
One caveat for London specifically: access. SIPs and modular panels are often large — some panels are full floor-to-ceiling height. If your garden access is through the house or via a narrow side passage, the delivery logistics need careful thought. Volumetric pods typically need a crane, which requires a road closure permit and advance coordination with the council.
Thermal performance
Modern SIPs panels outperform traditional cavity wall construction on U-values and airtightness. A 172mm SIPs panel typically achieves a U-value of 0.15–0.18 W/m²K. Traditional 100mm cavity wall construction with full-fill insulation typically achieves 0.18–0.22 W/m²K — achievable, but requiring more careful detailing.
The more significant advantage is airtightness. SIPs panels have fewer joints and thermal bridges than stick-built construction, and factory manufacture means tolerances are tighter. Achieving an air permeability of 3 m³/h/m² or better — a meaningful energy efficiency threshold — is easier with SIPs than with traditional masonry.
For London homeowners paying attention to energy performance certificates, heating bills, or compliance with the Future Homes Standard 2025, this is a genuine benefit.
Cost
Modular is not simply cheaper. The factory manufacturing and delivery cost is real, and in London — where labour costs are already high — the savings from reduced on-site labour are less dramatic than they are in other regions. A reasonable comparison for a standard single-storey rear extension in London in 2026:
| Build method | Cost per m² (London, inc VAT) | On-site programme |
|---|---|---|
| Traditional brick and block | £1,800–£2,800 | 8–16 weeks |
| Timber frame panel | £1,400–£2,000 | 4–8 weeks |
| SIPs panel system | £1,500–£2,200 | 3–6 weeks |
| Light gauge steel frame | £1,600–£2,400 | 3–6 weeks |
| Volumetric modular pod | £1,800–£2,800 | 1–3 weeks |
Costs are for the structural shell plus internal finishes, excluding foundations, services connections, glazing, and architect fees. All-in costs (adding architect £4,000–£10,000, structural engineer £1,500–£3,000, planning and building regs, contingency) typically add £8,000–£18,000 to any method.
Important: The cost per m² figures above are for the extension structure and finishes only. Foundations and ground works are the same regardless of build method — you will always need excavation, concrete, drainage, and damp-proof measures. Do not compare quotes that exclude foundations from one contractor and include them from another.
Quick Answer
Our AI can give you a cost estimate for a modular or traditional extension at your London property — enter your address to check planning constraints and get a realistic cost range.
£1,400–£2,000/m²
Timber frame
£1,500–£2,200/m²
SIPs panels
£1,800–£2,800/m²
Volumetric pod
Check your specific property constraints
Free Property CheckPlanning Permission: Same Rules as Traditional
The most important thing to understand about modular extensions and planning is that the construction method is irrelevant to planning law. A single-storey rear SIPs extension is assessed in exactly the same way as a single-storey rear brick extension. The question is whether the development requires planning permission, and the answer depends on the size, position, height, and property type — not the materials used to build it.
Permitted development: when it applies
A single-storey rear extension can be built under permitted development (Class A of Schedule 2, Part 1 of the GPDO 2015) without a planning application if it meets all the conditions. In summary for a terraced or semi-detached house:
- Maximum depth 3m from the rear wall (or 4m under prior approval via the Neighbour Consultation Scheme)
- Maximum height 4m (3m if within 2m of the boundary)
- Materials similar in appearance to the existing house
- No verandas, raised platforms, or balconies
- Not on a listed building or in a conservation area (for side extensions)
- Not subject to an Article 4 direction removing permitted development rights
The "materials similar in appearance" condition is worth noting for modular builds. A SIPs extension clad in brick slips, render, or timber cladding that matches the host dwelling can satisfy this condition. A modular pod with a highly contemporary zinc or Corten steel cladding may not — and in that case planning permission would be required regardless of whether the dimensions sit within permitted development limits.
When planning permission is needed
You need a planning application — the same £528 householder application as for any extension — if:
- The extension exceeds the permitted development size limits
- Your property is in a conservation area and the extension is visible from a road or public space
- Your property is a listed building (listed building consent also required in addition)
- An Article 4 direction has removed permitted development rights at your address
- The extension is a two-storey or wrap-around design
London note: A significant proportion of inner London properties are in conservation areas and/or subject to Article 4 directions. Do not assume permitted development applies because the size is within limits. Check planning constraints at your specific address before committing to a modular supplier.
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Building Regulations for Modular Extensions
All extensions — regardless of construction method — require building regulations approval. For modular systems, the route is the same as for traditional builds: either full plans approval submitted before work starts, or a building notice (more suitable for smaller projects where you are confident in compliance). The difference is that much of the compliance evidence for a modular system is produced by the manufacturer, not your contractor.
Third-party certification and type approval
Reputable SIPs and modular manufacturers typically hold third-party certification from bodies such as the British Board of Agrément (BBA) or NHBC Accepts. This certification documents that the system has been independently tested against the relevant parts of the Building Regulations. Building control will accept this as evidence of compliance for the structural and thermal performance of the panels themselves.
This does not mean building regulations are bypassed — it means the evidential burden is shared between the manufacturer's certification and your architect's drawings. Your structural engineer still needs to sign off foundations. Your building control surveyor still inspects at key stages.
Key building regulations considerations for modular extensions
Foundations must be designed to suit your specific site conditions. In London, clay subsoil is common and requires foundations at 1m–1.5m depth or deeper near trees. The panel system itself can use the manufacturer's certification for structural compliance, but foundations are always site-specific and require a structural engineer's input.
The 2022 Part L standards (which came into full effect for new extensions in 2023 and apply in 2026) require walls to achieve U-value 0.18 W/m²K or better, roofs 0.15 W/m²K, and floors 0.13 W/m²K. A well-specified SIPs panel meets the wall standard without additional insulation layers. Roof panels may need additional build-up depending on the specification. Check the manufacturer's compliance data against the current standards before ordering.
The high airtightness of SIPs construction is an energy efficiency advantage, but it means mechanical ventilation becomes more important. A SIPs extension kitchen or bathroom requires adequate mechanical extract. A whole-house ventilation strategy may be needed if the extension significantly reduces infiltration in the existing dwelling. This is often missed in early cost planning.
The external wall cladding on a modular extension must comply with fire performance requirements. This is particularly relevant for cladding materials on the external face of SIPs panels. Timber cladding, brick slips, and render all have different fire classifications — ensure your architect specifies compliant materials and that the building control submission addresses the full external wall build-up.
Where the modular extension meets the existing house is the most critical interface. Thermal bridges, airtightness continuity, and structural connection all need careful detailing at the junction. Factory-made systems are precise; the existing house is not. Allow time in the build programme for your contractor to manage the junction carefully — this is where modular extensions most commonly have post-build issues.
Unsure whether your modular extension needs planning permission or prior approval? Our AI checks your address and tells you the planning route instantly.
Check now — it's free →When Modular Works Best in London
Modular and off-site construction is not the right answer for every London extension. Understanding where it performs best helps you decide whether to go down this route.
Single-storey rear kitchen extension
The most common London extension type, and a good match for SIPs or timber frame. A single-storey rectangular or L-shaped rear extension is straightforward to manufacture as a panel system. The faster on-site programme reduces the time your kitchen is out of use — one of the biggest practical burdens of a traditional extension for a family home.
Garden room or home office pod
Volumetric modular pods are almost ideally suited to standalone garden room and home office structures. A factory-finished pod with insulation, electrics, and internal lining already installed can be placed in a rear garden in a single day with minimal foundations. This is the use case where the speed advantage is most dramatic and the access logistics (crane over the roof if needed) are most manageable.
Properties with difficult site access
In a London terrace with access only through the house, a long traditional build with daily deliveries of bricks, mortar, and sand becomes logistically challenging. A SIPs build has a concentrated delivery (the panels arrive once) and a faster programme that reduces the total number of contractor working days in difficult conditions. This is particularly relevant in dense inner-London streets where parking restrictions and narrow alleys make traditional builds especially disruptive.
Properties where energy performance is a priority
If you are planning to install a heat pump or aiming for a high EPC rating on your home, the superior airtightness of a SIPs or timber frame extension makes the heating system design more efficient. A well-detailed SIPs extension can achieve air permeability of 1–3 m³/h/m² — significantly better than most traditional masonry builds, which commonly test at 5–10 m³/h/m².
Where modular is less well-suited
Modular construction is harder to justify for:
- Complex geometries — curved walls, unusual plan shapes, or intricate roof forms are harder to prefabricate cost-effectively
- Projects where the architect's design changes significantly during the build — design must be fixed before manufacturing begins, unlike traditional build where some changes can be accommodated on site
- Listed buildings and conservation area properties where the planning authority requires specific traditional materials or detailing that must be executed on site
- Basement extensions, which require excavation, waterproofing, and underpinning regardless of superstructure method
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UK Modular Extension Suppliers and Manufacturers
The modular extension market in the UK has grown significantly. For London homeowners, the key is finding suppliers with experience of urban constraints — access logistics, conservation areas, and tight programme management around neighbours.
What to check when selecting a modular supplier
Ask for BBA (British Board of Agrément) certification or NHBC Accepts documentation for the panel system. This is the standard evidence that building control will accept for structural and thermal compliance. Suppliers without independent certification require more onerous documentation from your structural engineer and building control officer.
Ask for specific references from London projects. Access through terraced houses, crane lifts over properties, road closure permits, and coordination with conservation officers are London-specific skills. A supplier with predominantly rural or suburban experience may underestimate the logistical complexity of an inner-London site.
Find out whether the supplier works with your own architect or requires you to use their in-house design team. For planning purposes, your architect needs to produce the planning drawings regardless. The best modular suppliers work closely with external architects and provide detailed CAD files for the structural and thermal details. Suppliers who insist on controlling all design may not produce drawings in the format your local planning authority requires.
Modular systems should come with a manufacturer's structural warranty — typically 10–25 years depending on the system. Confirm what the warranty covers: panels only, or the full installation including junctions and weather seals? Who resolves issues if water ingress occurs at the panel-to-existing-house junction five years after completion?
BOPAS accreditation is increasingly sought by mortgage lenders as evidence that an off-site construction system has been assessed for durability, structural performance, and mortgageability. Suppliers with BOPAS accreditation significantly reduce potential mortgage complications when you come to sell or remortgage.
Mortgages and Finance: The Non-Standard Construction Question
This is the area that concerns London homeowners most — and where the landscape has shifted significantly in recent years. Historically, non-standard construction was a red flag for mortgage lenders. The picture in 2026 is more nuanced.
Is a modular extension non-standard construction?
It depends. A brick-faced SIPs extension that is indistinguishable externally from a traditional cavity wall extension is unlikely to cause any mortgage concern at all — the lender's surveyor will assess the external appearance and the building regulations completion certificate, not the method of construction. Most mainstream lenders are comfortable with this.
A fully modular volumetric extension with a highly contemporary material palette — particularly one that accounts for a significant proportion of the total property footprint — may prompt a surveyor to flag the construction method. This is more likely to affect resale and remortgage than the initial mortgage on the main property.
Reducing mortgage risk with modular construction
The Buildoffsite Property Assurance Scheme (BOPAS) provides lenders with independent evidence of a system's durability and structural performance over a 60-year lifespan. An increasing number of high street lenders accept BOPAS as sufficient to lend on MMC properties. Using a BOPAS-accredited supplier is the single most effective way to protect your mortgageability.
Keep the manufacturer's BBA or third-party certification, the building regulations completion certificate, the structural engineer's sign-off, and the manufacturer's warranty documentation. When you sell or remortgage, any surveyor querying the construction method needs to see this paperwork. Missing documentation creates unnecessary delay and potential down-valuation.
Your mortgage conditions almost certainly require you to notify your lender before carrying out significant works. This is true for any extension, modular or traditional. Get written acknowledgment from your lender before breaking ground.
Update your buildings insurance to cover the extension. Most standard policies cover SIPs and timber frame extensions without premium uplift, but confirm this with your insurer. If you have a retrofit guarantee or manufacturer's warranty on the panel system, check whether the insurer requires this to be registered or in force for the policy to remain valid.
Selling tip: When selling a property with a modular extension, proactively provide the buyer's solicitor with all the construction documentation before they ask. Properties with unusual construction methods where documentation is absent or late to arrive frequently cause delays or abortive transactions. Prepare a documentation pack at practical completion.
Frequently Asked Questions
Is a modular or SIPs extension treated the same as a traditional extension for planning permission?
Yes. Planning law is entirely indifferent to construction method. Whether you build a single-storey rear extension in traditional brick and block or SIPs panels, the same permitted development rules and planning application requirements apply. What matters is the size, height, position, and appearance of the extension — not what it is made of.
How much does a modular extension cost in London in 2026?
Typical costs in London inc VAT: timber frame panel system £1,400–£2,000/m²; SIPs panel system £1,500–£2,200/m²; light gauge steel frame £1,600–£2,400/m²; volumetric modular pod £1,800–£2,800/m². These figures are for the structural shell and finishes, excluding foundations, glazing, services connections, and architect fees. Traditional brick and block typically costs £1,800–£2,800/m² in London. Modular systems offer most cost advantage on speed and disruption rather than outright material cost.
How fast can a modular extension be built?
A SIPs or timber frame structural shell for a typical single-storey rear extension can be erected in two to five days on site. Total on-site programme from foundation pour to plastered walls typically runs three to six weeks, compared to eight to sixteen weeks for a traditional brick and block build of equivalent size. The speed advantage comes from factory manufacture happening concurrently with site preparation.
Will a SIPs or modular extension cause mortgage problems?
For most London homeowners, no — particularly if the extension is brick-faced or rendered to match the existing house. A modular extension indistinguishable from traditional construction rarely concerns lenders. A highly contemporary volumetric pod may prompt a surveyor to flag the method; using a BOPAS-accredited supplier and retaining all manufacturer certification significantly reduces this risk. Inform your mortgage lender before starting work as a standard condition of most mortgages.
Do I need a specialist builder for a SIPs extension?
The panel erection should be done by an experienced SIPs installer — ideally one recommended by or trained by the panel manufacturer. The groundworks, foundations, roofing, glazing, and internal finishes can be handled by a standard building contractor. A key advantage of SIPs is that the structural shell erection is fast, reducing the specialist contractor's programme on site. The most critical part requiring care is the junction between the new SIPs extension and the existing house.
What is BOPAS and why does it matter for a modular extension?
BOPAS (Buildoffsite Property Assurance Scheme) is an independent accreditation scheme that assesses off-site and modern methods of construction for durability, structural performance, and mortgageability over a minimum 60-year lifespan. An increasing number of mortgage lenders accept BOPAS accreditation as sufficient evidence to lend on properties with MMC components. Using a BOPAS-accredited modular supplier protects your ability to remortgage and sell the property in future without complications from non-standard construction queries.
Summary: Is Modular Right for Your London Extension?
Modular and off-site construction offers genuine advantages for London homeowners: faster build programmes, reduced on-site disruption, and superior thermal performance. These are not marginal gains — a six-week build versus a sixteen-week traditional build is a material quality-of-life difference for a family living through a kitchen extension.
The planning and building regulations position is straightforward: the same rules apply as for any extension. The construction method does not change what planning permission is needed, though the final appearance of the extension affects whether materials conditions are met under permitted development.
Mortgage and resale concerns are real but manageable. Use a supplier with BBA certification and BOPAS accreditation where possible, retain all documentation, and choose cladding and finishes that read as conventional from the street.
The best London extensions of the next decade will combine good architectural design with modern construction methods. Modular is increasingly one of the mainstream options — not a niche alternative.
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