Extension Above a Garage London 2026: Planning, Costs & Structural Guide
Building above a garage is one of the most underused ways to add a habitable room in London. Whether you have an integral garage under your first floor or a detached garage in the garden, the planning rules and structural challenges are very different — and often misunderstood.
Quick Answer
Costs inc VAT in London 2026: Converting an existing void above an integral garage costs £25k–£45k. Adding a new storey above an integral garage costs £55k–£90k. Adding a storey above a detached garage costs £65k–£110k. Detached garage upper storeys almost always need full planning permission — permitted development does not cover them.
£55k–£90k
Above integral (new storey)
£65k–£110k
Above detached garage
Essential
Structural survey
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Free Property CheckThe Two Very Different Scenarios
Before anything else, you need to identify which type of garage you have. The planning rules, structural implications, and costs diverge sharply depending on the answer.
Scenario A: Integral garage
An integral garage is built into the house footprint — typically at ground floor level with living accommodation above it on the first floor, or as a single-storey section at the side or front of the house. In London terraces and semis, the integral garage is usually at the front, accessed from the street, with a bedroom or landing directly above it.
There are two sub-scenarios here: (a) the house is already two-storey and there is an existing void or habitable room above the garage that can be improved or reconfigured, or (b) the integral garage is a single-storey element with no room above it, and you want to add a storey on top.
Scenario B: Detached garage
A detached garage sits in the garden, separate from the main house. It may be immediately adjacent to the rear or side of the house, or free-standing at the back of the plot. Adding a habitable upper floor to a detached garage almost always requires full planning permission — and is rarely covered by permitted development.
The connection to the main house is also a significant design question: an internal staircase through the garage, or a covered link structure bridging the gap.
The reason the distinction matters so much: permitted development rights for extensions to the house do not extend to adding a storey on top of a detached outbuilding. The PD rules for outbuildings cap height at 4m (with a dual-pitch roof) or 3m (flat/mono roof), and they cover the outbuilding structure as a whole — not a separate habitable floor above it.
Integral Garage: Adding a Storey Above
Converting an existing void (two-storey structure)
Many London semi-detached houses have an integral garage under the first floor, where the room above the garage has already been built but used as a bedroom, study, or storage room. In this case, you are not adding a new storey — you are improving or reconfiguring an existing floor plate. This is the simplest and least expensive option.
The structural work is limited: you may need to improve the floor structure (old garage floors were sometimes built to a lower specification than bedroom floors), upgrade insulation between the garage ceiling and the room above, and ensure fire separation is adequate. This often does not require planning permission and may not even require a building regulations application if no material changes to the structure are made.
Adding a storey on top of a single-storey integral garage
This is a more substantial project. You are raising the roof of the single-storey garage section to create a new habitable room at first-floor level, connecting it to the existing first floor of the house.
Planning position for integral garage — new storey above
Adding a storey on top of the integral garage section is treated as an extension to the house. Whether it needs planning permission depends on how the extension is classified:
- If the new room sits above the existing footprint (not extending into the garden or beyond the existing house volume), it may be treated as a loft-style conversion or internal works — and may not need planning permission.
- If adding the storey increases the overall height of the single-storey section to match or exceed the main ridge, it typically falls under Class AA permitted development (upward extensions) — which has its own conditions.
- If the property is in a conservation area, Article 4 direction area, or is a listed building, full planning permission is required regardless.
The precise planning route depends on your specific house configuration. A pre-application enquiry to the local planning authority, or a lawful development certificate application, is strongly recommended before committing to design.
Detached Garage: Planning Permission Almost Always Required
Adding a habitable upper storey to a detached garage in the garden sits outside the scope of permitted development as it is commonly understood. The PD rules for outbuildings (Class E) allow a single-storey structure up to 4m high (dual-pitch) or 3m (flat/mono roof) — they do not permit creating a habitable upper floor within or above an outbuilding.
In practice, a detached garage with a habitable room above will typically stand 5–6m tall, well above the outbuilding PD height limit. Full planning permission is therefore needed in virtually every case.
What councils assess for a detached garage upper storey
Height and massing
The council will assess whether the new upper storey is proportionate to the garden and the main house. A two-storey outbuilding at the bottom of a small London garden can look overbearing. The further the garage sits from the rear boundary, the better the outlook for neighbours and the stronger the planning case.
Impact on neighbours — overlooking and loss of light
Windows in an upper-storey outbuilding can directly overlook a neighbouring garden or rear windows. Planners will scrutinise window positions carefully. Obscure-glazed or high-set windows on elevations facing neighbours are often required as a condition of approval.
Ancillary use
Planning permission for a room above a garage is normally granted as ancillary accommodation — a home office, studio, or guest room — rather than a self-contained residential unit. Creating an independent dwelling (separate kitchen, bathroom, and sleeping space not connected to the main house) would require a separate planning consent for a new dwelling, which is much harder to obtain in garden land.
Design and materials
Councils will want the upper storey to be in keeping with the main house in terms of materials and architectural character. A brick or rendered upper storey with a pitched roof aligned with the main house roof pitch generally performs better in planning than a visually discordant structure.
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2026 Costs Inc VAT
All figures below include VAT at 20%. Construction cost is the build contract. All-in adds architect, structural engineer, building regulations, and a 10% contingency. Planning fees are not included where required (£528 for a full householder application in England).
| Scenario | Construction | All-in |
|---|---|---|
| Void above integral garage (conversion of existing space) | £20k–£38k | £25k–£45k |
| New storey above integral garage (single-storey section) | £46k–£75k | £55k–£90k |
| New storey above detached garage | £54k–£92k | £65k–£110k |
What drives the cost range
Foundation upgrades and covered link structures are the biggest unknowns. Both are identified at structural engineer assessment and design stage — not after work starts.
The detached garage scenario costs more than the integral because it typically requires more extensive foundation work (the garage was built as a single-storey structure with lighter foundations), and because connecting the upper room to the main house requires either a staircase through the garage or a separate covered link — adding significant cost and complexity.
Unsure whether your garage foundations can support a room above? Our AI can help you understand what a structural assessment involves and what questions to ask.
Ask now — it's free →Structural Requirements: The Critical First Step
The most common mistake people make when planning a room above a garage is assuming the existing structure can take the load. Garages — particularly detached garages built in the mid-twentieth century — were designed as single-storey utilitarian structures. Their foundations, walls, and roof structure are typically inadequate for a habitable upper floor without significant upgrading.
Foundation assessment
A structural engineer will carry out a foundation assessment — typically involving trial pits dug at the corners of the garage to inspect the existing foundation depth, width, and condition. In London clay, garage foundations are often only 450–600mm deep: adequate for a single-storey structure, but inadequate for the additional load of a habitable floor above. If the foundations are insufficient, the options are underpinning (expensive and disruptive) or reinforcing the existing structure with a new structural steel frame that transfers loads to new, deeper foundations at discrete points.
Wall structure
Single-skin brick walls (common in older garages) are not suitable as structural walls for a floor above. The structural engineer will specify either reinforcing the existing walls, replacing single-skin with cavity walls, or installing a structural steel or timber frame that the new floor deck and roof structure are supported by independently of the existing garage walls.
New floor deck
The garage roof is removed and replaced with a structural floor deck — typically an engineered timber or concrete composite floor — that forms the base of the new room. The structural engineer sizes this to carry habitable floor loads (the residential imposed load standard is 1.5 kN/m²) including furniture, occupants, and any acoustic or insulation layers. This floor deck also forms part of the fire separation between the garage and the room above (see building regulations section).
New roof structure
Once a habitable floor is added above, the roof is new. This is typically a pitched roof structure (to match the main house in planning terms) or, where permitted, a flat roof. The new roof must meet current building regulations thermal performance standards (U-value 0.15 W/m²K or better).
Bottom line on structure: Commission a structural engineer assessment before progressing any design work. The cost is £500–£1,200 for the initial assessment. Without it, you cannot know whether the project is feasible at a sensible cost, or whether you are looking at a straightforward new-build above a stable existing structure.
Building Regulations for a Habitable Room Above a Garage
Building regulations approval is required regardless of planning status. The key requirements specific to a room above a garage are thermal performance, fire separation, and means of escape.
Thermal insulation (2026 standards)
- Floor (between garage and room above): U-value 0.13 W/m²K or better — this is the most demanding element of the project, as the floor must achieve high performance while incorporating structural and fire-resistance layers
- Walls: U-value 0.18 W/m²K or better
- Roof: U-value 0.15 W/m²K or better
- Glazing: U-value 1.4 W/m²K or better
The floor insulation requirement is particularly important because the garage below is unheated. Heat loss through the floor is significant if this is not addressed properly.
Fire separation between garage and living space
This is a requirement that is frequently underestimated on garage conversion projects and above-garage rooms alike. Building regulations (Approved Document B) require 30-minute fire resistance between a garage and any attached or adjacent habitable accommodation. In practice this means:
- The floor between garage and room above must achieve 30-minute fire resistance (FR30) — typically achieved with two layers of 12.5mm fire-rated plasterboard, or equivalent structural fire-rated board
- Any door between the garage and the habitable area must be a fire door — FD30 self-closing fire door with intumescent strip
- Penetrations through the fire-separating floor (pipes, cables, drainage) must be fire-stopped with intumescent collars or fire-rated seals
- The garage floor should be raised 100mm above the garage entrance threshold to prevent fuel spills entering the building structure
Means of escape
The room above a garage must have a safe route of escape in case of fire. If the room connects to the main house via an internal staircase, that staircase forms part of the protected escape route and must be enclosed in fire-rated construction. If the room is in a detached garage and accessed only via an external stair, a second means of escape (such as a window of sufficient opening area for escape or rescue) may be required.
Ventilation and electrics
Habitable rooms require adequate ventilation: a background ventilator of at least 8000mm² equivalent area and a rapid purge ventilator (openable window of at least 1/20 of floor area). Any new electrical installation must comply with Part P and be certified by a registered electrician.
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Party Wall: When It Applies
The Party Wall Act 1996 is triggered by excavation works within 3m of a neighbouring building and by work to a shared wall on or near the boundary. For a room above a garage, the most likely trigger is:
- Foundation excavations within 3m of a neighbouring structure — very common for an integral garage that shares a party wall with the adjoining house
- The garage wall is on or along the boundary with a neighbouring property
- Any structural work to or through a party wall shared with a neighbour
Cost: Budget £800–£1,400 per affected neighbour. If the garage is bounded on two sides by different neighbours, you may need to serve notice on both.
Timing: Serve notice at least 2 months before work starts. If neighbours dissent or do not respond within 14 days, a party wall surveyor must be appointed. Build party wall timelines into your project programme from the outset.
Access: Connecting the Room to the Main House
How you access the room above the garage is one of the most important design decisions in the project. It affects cost, planning, building regulations, and usability.
Internal staircase through the garage
- No external exposure to weather
- Lower cost than a covered link
- Works for integral and some detached garages adjacent to house
- Uses garage floor space for stair and landing
- Fire separation at stair opening is a building regulations requirement
Typical cost: £3,500–£8,000 for the staircase
Covered link to first floor of main house
- Preserves full garage floor area for parking or storage
- Connects directly to main house first floor without going through garage
- Significantly higher cost
- Usually requires planning permission as a separate structure
Typical cost: £12,000–£28,000 for the link structure
For a detached garage that is not immediately adjacent to the house, a covered link is often the only practical way to make the room genuinely usable year-round. However, the link itself is a separate structure that needs planning permission and building regulations approval, adding cost and programme time. Some clients opt for an external stair and accept that access is outdoor — appropriate for a summer studio but less practical for a year-round office or bedroom.
Best Uses for a Room Above a Garage
The most successful uses are those that work with the inherent characteristics of the space — slightly removed from the main house, typically with good natural light on at least one side, and insulated from the sounds of family life.
Home office
The most popular use. The separation from the main house is an asset — it creates a genuine work-from-home environment. Good broadband and power supply are essential (both straightforward if designed in from the start).
Guest bedroom or teenager's room
A bedroom above the garage works well as a semi-independent space. Ensure it has an en-suite or easy access to a bathroom — an afterthought on this can undermine the usability of the space. Fire escape from the bedroom is a building regulations requirement to verify at design stage.
Art studio or music room
The acoustic separation between garage and room above (particularly if the fire-rated floor deck uses acoustic insulation) makes this a good candidate for music practice. North-facing rooflights are ideal for an art studio with consistent natural light.
Gym
A gym above a garage avoids impact noise transferring to living areas in the main house. The structural floor deck specification needs to account for dynamic loading from exercise equipment — flag this with your structural engineer at the outset.
Common Mistakes
Assuming permitted development covers a detached garage upper storey
This is the most frequent planning error. Permitted development for outbuildings (Class E) does not permit habitable upper floors in detached outbuildings. If you build without permission and the council issues an enforcement notice, you face either demolition or a retrospective planning application — neither is pleasant or cheap.
Skipping the structural survey
Proceeding to planning and design without a structural assessment means you may design a project that is physically feasible but financially unviable once foundation upgrade costs are known. Commission the structural engineer assessment before the architect produces detailed drawings — not after.
Overlooking fire separation
Fire separation between a garage (which stores flammable liquids and vehicles) and a habitable room above is a safety-critical building regulation requirement. It is occasionally omitted on self-build or poorly managed projects. Building control will require evidence of compliance at inspection — make sure it is in the specification from day one, not added as an afterthought.
Underestimating the link structure cost
A covered link between a detached garage upper floor and the main house is often treated as a minor element in early budgets. In practice it is a full building project in miniature: foundations, walls, roof, waterproofing, insulation, glazing, planning, and building regulations. Budget £12,000–£28,000 and allow 8–10 weeks of planning and design time for the link alone.
Treating the room as ancillary when planning expects ancillary use
If you obtain planning permission for a home office or studio above a garage and then fit it out as a self-contained flat with a kitchen and shower room, you are in breach of the planning permission. The council can enforce against this. If you want a self-contained annex, apply for that from the outset — it is a harder permission to obtain but it is the lawful route.
Frequently Asked Questions
Do I need planning permission to build above my garage in London?
It depends on the garage type. Adding a habitable upper storey to a detached garage almost always requires full planning permission — permitted development for outbuildings does not cover habitable upper floors. For an integral garage that is part of the main house, the position is more nuanced and may not need planning permission if the work stays within the existing house footprint. Always check with the local planning authority or apply for a lawful development certificate before proceeding.
How much does a room above a garage cost in London in 2026?
All-in costs inc VAT: converting an existing void above an integral garage £25k–£45k; new storey above an integral garage (single-storey section) £55k–£90k; new storey above a detached garage £65k–£110k. Key variables are whether foundation upgrades are required and whether a covered link to the main house is needed. Commission a structural engineer assessment before finalising your budget.
Do garage foundations need upgrading to support a room above?
Very often yes, particularly for detached garages built before 1990. Garage foundations were designed for a single-storey structure and are typically too shallow for the additional load of a habitable floor. A structural engineer assessment (£500–£1,200) will confirm whether underpinning or new foundations are needed. This is the most important step before committing to any design or budget.
What fire separation is needed between a garage and a room above?
Building regulations require 30-minute fire resistance between a garage and any adjacent habitable accommodation. In practice: the floor deck between garage and room must achieve FR30 (typically two layers of 12.5mm fire-rated plasterboard or equivalent); any door between garage and habitable area must be a self-closing FD30 fire door with intumescent strip; all penetrations (pipes, cables) through the fire-separating floor must be fire-stopped. This applies whether the garage is integral or detached.
Can I use a room above a detached garage as a separate flat?
No, not without a specific planning permission for a new residential dwelling. Planning permission for a room above a garage is normally granted as ancillary accommodation (home office, studio, guest room) ancillary to the main house. If you want a self-contained flat, you need to apply for that use from the outset — it is considerably harder to obtain as it creates a new separate dwelling in garden land.
What is the best way to connect a room above a detached garage to the main house?
Either an internal staircase through the garage, or a covered link structure connecting the upper floor directly to the main house first floor. An internal stair through the garage preserves the connection without outdoor exposure but takes floor space. A covered link is more expensive (£12k–£28k) but preserves the full garage floor and connects directly to the house. Both options need building regulations approval; a covered link also typically needs planning permission.
Does building above a garage trigger the Party Wall Act?
Potentially yes. If the garage is adjacent to or shares a wall with a neighbouring property, or if the foundation works are within 3m of a neighbouring structure, the Party Wall Act 1996 is triggered. Serve notice at least 2 months before work starts. Budget £800–£1,400 per affected neighbour for surveyor fees.
Summary
A room above a garage can add a genuinely useful space to a London home — particularly where garden space is limited and ground floor extensions are not viable. The key to making the project work is getting the sequence right.
Start with the structural engineer assessment. This tells you whether the project is feasible and what the foundation and structural upgrade costs are — information that drives the whole budget. Then establish the planning position: integral garage works are more likely to be permitted development or fall within Class AA; detached garage upper storeys almost always need full planning permission.
Fire separation between the garage and the room above is non-negotiable and must be in the specification from day one. Do not leave it for building control to pick up as a snag — it is safety-critical and should be designed in, not added on.
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