Flat Extension London 2026: Planning, Costs & Leasehold Guide
Extending a flat in London is possible but works very differently from extending a house. Flats have no permitted development rights, you need your freeholder's consent before you even think about planning, and the building regulations are stricter. Here is what flat owners actually need to know.
Quick Answer
Flats have NO permitted development rights for extensions. You always need planning permission and freeholder consent. Ground floor flat extensions cost £55k–£130k+ inc VAT depending on size. The process starts with checking your lease and getting a licence to alter from the freeholder — not with a planning application.
£55k–£80k
Small extension (12–15m²)
£75k–£120k
Medium extension (16–24m²)
Always
Planning required?
Check your specific property constraints
Free Property CheckCan You Actually Extend a Flat in London?
Yes, but with significant caveats. The most important thing to understand is that flats and maisonettes do not have permitted development rights for any external changes. This catches many flat owners off guard. The generous PD allowances that let house owners build 3–6m rear extensions without planning permission simply do not apply to flats.
Every external alteration to a flat — whether that is a rear extension, a new window, or even replacing an external door — requires a planning application to the local council. There are no exceptions, regardless of how small the change or what borough you are in.
Which flats can realistically be extended?
Ground floor flats
The best candidates. A ground floor flat with access to a private rear garden can be extended outward in the same way as a house — rear extension, side return infill, or both. This is by far the most common flat extension scenario in London.
Ground floor maisonettes
Maisonettes that occupy the ground and lower ground floors have even more flexibility. You can extend at ground level and potentially excavate to create a larger lower ground floor. Many Victorian and Edwardian conversions fall into this category.
Upper floor flats
Very limited options. A top-floor flat might be able to add a dormer loft conversion if the lease includes the roof space. Middle-floor flats are essentially restricted to internal reconfiguration only. External extension is not realistic for upper floors.
The rest of this guide focuses primarily on ground floor flat extensions, which are the most viable and most commonly built in London.
Leasehold: The Step Most People Miss
Before you commission an architect or submit a planning application, you need to deal with the lease. Most London flats are leasehold, which means you do not own the building — you own the right to occupy your flat for a fixed term. The freeholder owns the structure and the land.
The licence to alter
Any structural alteration to a leasehold flat requires formal written consent from the freeholder. This consent takes the form of a licence to alter — a legal document that describes the proposed works, sets conditions for how they must be carried out, and typically requires you to reinstate the property at the end of the lease if asked.
What the licence to alter process involves
Total timeline: 2–4 months. The freeholder cannot unreasonably withhold consent (Landlord and Tenant Act 1927, s.19), but they can set conditions and charge you for their surveyor and legal costs. Budget £1,000–£3,000 for the freeholder's professional fees.
Key lease terms to check
- Demise boundary: Your lease defines exactly what you own. Typically the internal surfaces of walls, floors, and ceilings. The external walls, roof, and structure belong to the freeholder. An extension changes the building envelope, which is why consent is essential.
- Garden rights: Check whether you have exclusive use of the garden or shared use. An extension built on shared garden space requires agreement from all leaseholders.
- Service charge impact: An extension increases your flat's floor area and may increase your proportion of service charge contributions. Some leases recalculate proportions automatically; others require a deed of variation.
- Insurance: The freeholder typically insures the building. An extension changes the building's footprint and value. The freeholder will require you to notify their insurer and may pass on any premium increase.
Leasehold reform update (2026): The Leasehold and Freehold Reform Act 2024 received Royal Assent in May 2024. It extends standard lease extensions to 990 years and reduces ground rents to a peppercorn. However, many provisions are being implemented in stages and the requirement for freeholder consent to structural alterations remains unchanged. You still need a licence to alter.
Share of freehold? If you collectively own the freehold with other leaseholders, you still need to follow the lease terms. The consent process is usually simpler but the legal requirements are the same.
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Planning Permission for Flat Extensions
Since flats have no permitted development rights for external alterations, every flat extension requires a full planning application. There is no prior approval shortcut. The good news is that councils approve the majority of well-designed ground floor flat extensions, particularly where the extension mirrors what would be acceptable on a house.
What councils assess
- Impact on the character and appearance of the building and street scene
- Effect on neighbours — light, outlook, privacy, and noise
- Whether the extension is subordinate to the original building
- Materials matching or complementing the existing building
- Remaining private outdoor amenity space
Common refusal reasons for flat extensions
Breaking the rear building line
In a converted building with multiple flats, councils are wary of one ground floor flat extending beyond the rear wall when the upper floors cannot. This creates an asymmetry that some boroughs resist, particularly where the rear elevation is visible from public areas.
Loss of light to upper floor flats
A ground floor extension with a flat roof can block light to the flat above, especially if the upper flat has windows looking down onto the area being built over. Councils may require a daylight and sunlight assessment.
Excessive coverage of garden space
Councils expect a reasonable amount of private outdoor space to remain. If the garden is small and shared, building over a significant portion may be refused. Most boroughs want at least 50m² of usable garden remaining per unit.
Planning application details for flats
Pre-application advice is strongly recommended for flat extensions. It gives you the council's informal view before you invest in full drawings and the licence to alter process.
Not sure if your flat is in a conservation area or subject to Article 4 restrictions? Our AI checks your address instantly.
Check now — it's free →Conservation Areas and Article 4 Directions
A large proportion of London's flats are in conservation areas. Many of the most desirable Victorian and Edwardian conversions sit within them. For flat extensions, the conservation area status adds another layer of scrutiny but does not necessarily prevent an extension.
Since flats already need full planning permission regardless, being in a conservation area does not change the application route. What it does change is the design standard expected. Councils will require the extension to preserve or enhance the character of the conservation area. In practice this means:
- Materials must match or be sympathetic to the existing building (London stock brick, slate, timber sash profiles)
- The extension must be clearly subordinate and not dominate the original building
- Roof form may need to be pitched rather than flat if visible from the street or public areas
- A heritage statement may be required explaining the design rationale
Article 4 directions: Some conservation areas have Article 4 directions that remove even basic rights like changing windows or external finishes without permission. For flats this is less relevant (since flats already need permission for everything), but Article 4 areas signal a borough that is particularly protective of the built environment. Expect stricter design scrutiny.
Listed buildings: If your flat is in a listed building, listed building consent is required in addition to planning permission. Alterations to listed buildings are held to a much higher standard and internal changes may also need consent. Specialist heritage architects are strongly recommended.
2026 Costs for Flat Extensions
Construction costs for a ground floor flat extension are broadly similar to a house extension of the same size. The premium comes from the additional legal, professional, and compliance costs that flat extensions carry.
| Size | Floor area | Construction | All-in |
|---|---|---|---|
| 3m × 4m | 12m² | £42k–£60k | £55k–£78k |
| 3m × 5m | 15m² | £52k–£75k | £68k–£96k |
| 4m × 5m | 20m² | £68k–£98k | £88k–£128k |
| 4m × 6m | 24m² | £82k–£118k | £106k–£152k |
Flat-specific additional costs
These are costs that flat extensions incur on top of the standard construction and professional fees that house extensions also carry:
All figures inc VAT at 20%. These flat-specific costs add £7,000–£18,000 to a project compared to an equivalent house extension. This is why the “all-in” column above is higher than the equivalent house extension costs.
Building Regulations: Where Flats Differ
All extensions need building regulations approval. For flat extensions, two areas require significantly more attention than house extensions: fire safety and sound insulation.
Sound insulation (Part E)
Because your extension shares a building with other flats, the separating floors and walls must meet Part E sound insulation standards. For conversions and extensions this means:
- Airborne sound: minimum 43dB through separating floors and walls
- Impact sound: maximum 64dB through separating floors
- Pre-completion testing by an accredited body is mandatory
- Results must be submitted to building control before sign-off
This typically means upgrading the ceiling construction of the extension where the flat above has habitable rooms, adding acoustic insulation and resilient bars or an independent ceiling system.
Fire safety (Part B)
Flats have stricter fire safety requirements than houses because multiple households share the building:
- Minimum 30-minute fire resistance for separating floors and walls (60 minutes for buildings over 3 storeys)
- Protected means of escape from each flat to the building exit
- Fire doors to FD30S standard on all flat entrance doors
- The extension must not compromise the existing escape routes for other flats in the building
- Interlinked smoke and heat alarms compliant with BS 5839 Part 6
Structural and thermal standards
The same standards apply as for house extensions: foundations designed by a structural engineer, U-values meeting 2026 standards (walls 0.18 W/m²K, roof 0.15 W/m²K, floor 0.13 W/m²K), and all electrical work to Part P. The structural engineer must also assess the impact of the new foundations and any structural openings on the existing building, which is shared with other leaseholders.
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The Process Step by Step
The critical difference from a house extension is the order: leasehold consent comes first, not planning. Here is the realistic sequence for a ground floor flat extension in London:
1. Check your lease and instruct a solicitor
Week 1–2Read the lease carefully or have a solicitor review it. Identify the alteration clause, your demise boundary, garden rights, and any restrictions on external changes.
2. Design and pre-application
Week 2–7Commission an architect. Get pre-application advice from the council (£150–£600). This tells you informally whether the council is likely to approve the design before you spend money on full drawings.
3. Licence to alter application
Week 4–16Submit the licence to alter application to the freeholder with your drawings and specification. This runs in parallel with planning. Most freeholders take 2–4 months to grant a licence.
4. Planning application
Week 6–18Submit the full planning application. Decision in 8–13 weeks. You do not need the licence to alter before submitting planning, but you need both before starting construction.
5. Building regulations and party wall
Week 10–22Submit building regulations full plans. Serve party wall notices on affected neighbours (both within and outside the building). These run in parallel with planning and leasehold.
6. Tender and appoint a contractor
Week 18–24Get at least three quotes from contractors experienced with flat extensions. A contractor who has worked on flats understands the access restrictions, party wall requirements, and need to minimise disruption to other residents.
7. Construction
Week 24–3810–16 weeks on site for a typical ground floor extension. Coordinate with other residents on access, working hours, and noise management.
Total project
6–10 monthsFlat extensions take 2–3 months longer than equivalent house extensions because of the licence to alter process and the need to coordinate with the freeholder and other residents.
Common Mistakes with Flat Extensions
Assuming permitted development rights apply
This is the single most common mistake. A flat owner reads about 3–6m rear extensions under PD and assumes the same applies to them. It does not. Flats have no PD rights for external changes. Every external alteration needs a planning application. Starting work without planning permission is an enforcement risk.
Not checking the lease first
Some leases prohibit all structural alterations outright. Others restrict extensions to specific areas or require specific materials. If you spend £5,000 on architectural drawings only to discover the lease prohibits external extensions, that money is wasted. Read the lease before commissioning any design work.
Not involving the freeholder early enough
The licence to alter process takes 2–4 months. If you wait until after planning approval to approach the freeholder, you add months to the project. Begin the freeholder conversation at the same time as the design process, not after.
Forgetting about the upper floor flat
A ground floor extension built directly below the upper flat's windows can significantly reduce their light. Councils assess this and neighbours in the building above will likely object. Design the extension to minimise impact on upper flats — consider roof lanterns or skylights that sit away from the upper floor windows.
Underestimating the sound insulation requirement
Part E sound testing is mandatory and the extension will not receive a building control completion certificate without passing. Specify the acoustic build-up during the design stage, not as an afterthought. Failing the pre-completion test means remedial work that can cost £2,000–£5,000.
Frequently Asked Questions
Can you extend a flat in London?
Yes, but only with planning permission and freeholder consent. Flats have no permitted development rights for external alterations. Ground floor flats with private garden access are the most viable candidates. Upper floor flats are limited to internal reconfiguration or, for top-floor flats, a possible loft conversion if the lease includes the roof space.
How much does a ground floor flat extension cost in London?
All costs inc VAT: 3m×4m (12m²) £55k–£78k all-in, 3m×5m (15m²) £68k–£96k all-in, 4m×5m (20m²) £88k–£128k all-in. Flat extensions cost £7,000–£18,000 more than equivalent house extensions due to licence to alter fees, enhanced sound insulation, fire safety upgrades, and solicitor costs.
Do I need planning permission to extend a flat?
Yes, always. Flats and maisonettes have no permitted development rights for external changes. Every external alteration including extensions, new windows, and even replacing an external door requires a planning application. There is no prior approval or permitted development shortcut for flats.
What is a licence to alter for a flat extension?
A licence to alter is formal written consent from the freeholder (landlord) allowing you to make structural alterations to the building. It describes the proposed works, sets conditions for construction quality, and typically requires reinstatement at lease end if requested. The freeholder cannot unreasonably withhold consent under the Landlord and Tenant Act 1927. Budget 2–4 months and £1,000–£3,000 for the freeholder’s professional fees.
Can you extend a flat in a conservation area in London?
Yes, but with stricter design requirements. Since flats already need full planning permission regardless, being in a conservation area does not change the application route. It does mean the council will scrutinise materials, design, and impact on the conservation area character more closely. A heritage statement may be required with the application.
How long does a flat extension take in London?
Total project timeline is 6–10 months. This breaks down as: lease review and design 2–7 weeks, licence to alter 2–4 months, planning application 8–13 weeks, building regulations and party wall 4–8 weeks (in parallel), and construction 10–16 weeks on site. Flat extensions take 2–3 months longer than house extensions due to the leasehold process.
Can I extend an upper floor flat in London?
External extension of upper floor flats is generally not possible. A top-floor flat may be able to convert the loft space if the lease includes the roof void, but this requires freeholder consent, planning permission, and structural assessment. Mid-floor flats are limited to internal reconfiguration only.
What building regulations apply to flat extensions?
Flat extensions must meet the same building regulations as house extensions (structural, thermal, drainage) plus enhanced requirements for sound insulation (Part E: 43dB airborne, 64dB impact through separating floors) and fire safety (Part B: 30-minute fire resistance, protected escape routes, FD30S fire doors). Pre-completion sound testing is mandatory.
Summary
Extending a flat in London is absolutely possible, but the process is more complex than extending a house. The two critical differences are the leasehold requirement (licence to alter from the freeholder) and the absence of permitted development rights (planning permission is always needed).
Ground floor flats with private gardens are the best candidates. The costs are broadly similar to house extensions with an additional £7,000–£18,000 for flat-specific legal and compliance requirements. The project takes 2–3 months longer than a house extension.
Start with the lease. If the lease allows alterations (most do, with freeholder consent), the rest of the process — design, planning, building regulations, construction — follows a well-established path. The key is doing things in the right order and involving the freeholder from the outset.
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