Most garages in West London have not seen a car in years. If yours is full of boxes and bikes, converting it is the cheapest way to add a proper room to your house – no foundations, no roof, no lost garden. This guide from GSM Builders Ltd, builders based in Hayes, sets out realistic garage conversion costs for 2026, the planning and Building Regulations rules, the ideas that work best, and how long it all takes.
Garage conversion cost: the quick answer
A single attached garage conversion in West London typically costs £15,000 to £25,000 for a fully finished, Building Control-certified room. Add plumbing for a WC or en-suite and you are looking at £20,000 to £32,000. A double garage or a detached garage costs more because there is more wall, floor and roof to insulate. Per square metre, expect £1,100 to £1,700 – roughly half the cost of building an extension of the same size.
| Garage conversion type | Typical size | 2026 cost range (ex VAT) | Build time |
|---|---|---|---|
| Single attached garage – basic room (office, snug, playroom) | 12–15 m² | £15,000 – £22,000 | 3 weeks |
| Single garage with WC or utility plumbing | 12–15 m² | £18,000 – £26,000 | 3–4 weeks |
| Single garage – bedroom with en-suite / wet room | 12–15 m² | £22,000 – £32,000 | 4–5 weeks |
| Partial conversion (front third kept for storage) | 8–10 m² | £10,000 – £16,000 | 2–3 weeks |
| Double garage – one large room or two rooms | 25–30 m² | £28,000 – £42,000 | 4–6 weeks |
| Detached garage – annexe, studio or office | 15–30 m² | £25,000 – £45,000 | 5–8 weeks |
Ranges are based on the garage conversions GSM Builders Ltd quotes across Hillingdon, Ealing, Hounslow and Harrow. Every garage is different – ask us for a free itemised quote.
What is included in the price?
A proper garage conversion is not a case of plasterboarding the walls and laying a carpet. To be signed off by Building Control – and to be warm, dry and worth money when you sell – the conversion needs:
- Removing the garage door and building a new insulated front wall in matching brick, with a window (or a set of doors) and a new lintel where needed
- A new floor – the garage slab is usually 100–150 mm below the house floor level, so we add a damp-proof membrane, rigid insulation and a screed or timber floor to bring it level with the hallway
- Wall insulation – a stud wall with insulation and a vapour barrier inside the single-skin brick walls
- Roof or ceiling insulation – flat roofs are insulated from below or above; pitched roofs at ceiling level
- Damp-proofing to walls where required
- Electrics – sockets, lighting, and a heating circuit, certified under Part P
- Heating – radiators off the existing system or underfloor heating
- Ventilation – trickle vents and, for bathrooms, an extractor fan
- Plastering, skirting, doors and decoration
- Building Control inspections and the completion certificate
If the conversion is going to be a bedroom, Building Regulations also require a fire escape route – normally an escape window – and mains-linked smoke alarms.
What affects the cost of a garage conversion?
1. Floor level and floor build-up
The bigger the step down into the garage, the more insulation and screed is needed to level the floor. Some 1930s garages in Hayes and Ruislip also have a sloping slab that has to be made level first.
2. The new front wall
Infilling the door opening in matching brick, with proper foundations under the new wall, is one of the larger single items. A wide picture window or French doors cost more than a standard window, but they make the room feel like part of the house.
3. Damp and condition of the structure
Single-skin garages often show damp at low level. Tanking or a damp-proof course adds £800–£2,000 depending on the extent.
4. Roof
Old felt flat roofs frequently need replacing at the same time as the conversion – budget £2,500–£4,500 for a new insulated flat roof on a single garage.
5. Plumbing
A WC, utility or en-suite means running hot and cold water and, crucially, a soil connection to the drains. If the nearest drain is on the other side of the house, that is extra excavation.
6. Structural work
Knocking through from the hallway or living room into the garage needs a structural opening with a steel or concrete lintel and calculations from an engineer.
7. Finish level
Underfloor heating, built-in storage, a fitted desk or a luxury wet room all push the price towards the top of the ranges above.
Garage conversion ideas that work in West London homes
Home office or study
The most requested conversion since 2020. A garage sits away from the noise of the kitchen and living room, and with a good window and proper insulation it becomes a quiet, year-round workspace. Add a second door to the garden for a separate entrance if clients visit.
Playroom or snug
Families in Hounslow, Greenford and Harrow often want a second living space where the children can make a mess. A garage conversion off the hallway is ideal, and it can become a teenager’s den or a downstairs bedroom later.
Home gym
A gym is one of the cheapest conversions because it needs little more than insulation, a rubber floor, good lighting and ventilation. Keep the ceiling as high as possible and add a mirror wall.
Guest bedroom with en-suite
Adding a downstairs bedroom and shower room is the conversion that adds the most value, and it is exactly what many multi-generational households in Southall, Hayes and Hillingdon are looking for. The en-suite pushes the cost up, but a bedroom with its own bathroom is a very different proposition for buyers than a plain extra room.
Utility room and downstairs WC
If you only need part of the garage, converting the back half into a utility room and cloakroom – and keeping the front for bikes and storage – is a practical compromise that costs £10,000–£16,000.
Self-contained annexe
Detached and double garages can become an annexe for an elderly parent or an adult child, with a bedroom, shower room and kitchenette. Annexes usually need planning permission and are treated differently for council tax, so speak to us early.
Do you need planning permission for a garage conversion?
In most cases, no. Converting an attached garage into living space is normally permitted development as long as the work is internal and the external appearance is not changed significantly – replacing the garage door with a matching brick wall and window is generally fine. You need planning permission if:
- Your house is in a conservation area with an Article 4 direction – parts of Ealing, Acton, Harrow on the Hill, Ruislip Village and Old Uxbridge
- The original planning permission for the house had a condition requiring the garage to be kept for parking – common on estates built since the 1980s, and something Hillingdon and Hounslow councils check because of parking pressure near Heathrow
- You live in a flat, maisonette or listed building
- The garage is detached and you want to use it as separate accommodation (this may be a change of use)
- You are extending the garage or altering the roof at the same time
Whichever applies, we recommend applying to your council for a Lawful Development Certificate (about £129 in 2026). It proves the conversion was legal, which your buyer’s solicitor will ask about when you sell. GSM Builders Ltd handles this paperwork with Hillingdon, Ealing, Hounslow, Harrow and Slough councils as part of the project.
Building Regulations for garage conversions
Building Regulations approval is always required for a garage conversion, because you are changing a non-habitable space into a habitable one. Building Control will check:
- Thermal performance – new walls, floor and roof must meet current insulation standards (roughly 0.28 W/m²K for walls, 0.18–0.25 for floors and 0.16 for roofs)
- Damp-proofing of the floor and walls
- Structure – the new lintel over the old door opening and any knock-through into the house
- Ventilation – background trickle vents plus mechanical extraction in any bathroom or kitchen
- Fire safety – an escape window if the room is a bedroom, and mains-wired smoke alarms linked to the rest of the house
- Electrics certified under Part P
- Drainage for any new WC or shower
We book the inspections, meet the inspector on site and hand you the completion certificate at the end. Without that certificate, a mortgage valuer will not count the room as habitable.
How long does a garage conversion take?
- Site visit and quote: within a few days of your enquiry
- Drawings and Building Regulations application: 2–3 weeks
- Lawful Development Certificate (optional): up to 8 weeks, but the build can start before it is issued
- Build: 3–5 weeks for a single garage; up to 8 weeks for a double or detached garage with plumbing
Because the work is mostly inside the garage, disruption to the rest of the house is minimal – most families stay at home throughout.
Does a garage conversion add value?
Estate agents across West London generally put the uplift from a well-finished garage conversion at 5–10% of the property value, and more if it creates a bedroom with an en-suite. On a £550,000 semi in Hayes or Greenford that is £27,000–£55,000 for a job costing £18,000–£28,000. The one caution: keep the driveway. Off-street parking is valuable in Hillingdon, Hounslow and Harrow, and a conversion that removes the drive as well as the garage can cancel out the gain.
Garage conversions in West London with GSM Builders Ltd
GSM Builders Ltd is a family-run building company at Leven Way, Hayes UB3. We convert garages across Hayes, Hillingdon, Uxbridge, Ruislip, Southall, Greenford and Northolt, Ealing, Hounslow, Harrow, West Drayton and Slough. One team handles the drawings, Building Control, all trades and decoration, with a written itemised quote, agreed payment stages and a 12-month workmanship warranty.
If a garage conversion will not give you enough space, we also build house extensions and loft conversions – see our extension cost guide and loft conversion cost guide to compare.
Garage conversion FAQs
How much does a garage conversion cost in the UK in 2026?
A single attached garage conversion typically costs £15,000–£25,000 for a finished, Building Control-certified room, or £20,000–£32,000 with a WC or en-suite. Double and detached garages cost £25,000–£45,000. Prices in West London sit towards the middle of national ranges; GSM Builders Ltd provides free itemised quotes.
Do I need planning permission to convert my garage?
Usually not – converting an attached garage is normally permitted development. You will need permission if your home is in a conservation area with an Article 4 direction, if the original planning consent requires the garage to be kept for parking, if you live in a flat or listed building, or if a detached garage is becoming separate accommodation. A Lawful Development Certificate is recommended in every case.
Does a garage conversion need Building Regulations approval?
Yes, always. Converting a garage to a habitable room requires Building Regulations approval covering insulation, damp-proofing, structure, ventilation, fire safety, electrics and drainage. You receive a completion certificate at the end, which you will need when you sell or remortgage.
How long does a garage conversion take?
A single garage conversion takes around 3–5 weeks on site, plus 2–3 weeks beforehand for drawings and the Building Regulations application. Double or detached garages with plumbing can take up to 8 weeks.
Does converting a garage add value to a house?
A well-finished garage conversion typically adds 5–10% to a property’s value in West London, and more if it creates a bedroom with an en-suite. Keep the driveway so you do not lose off-street parking, which buyers value highly.
Can a garage be converted into a bedroom?
Yes. A garage can become a bedroom provided it meets Building Regulations for insulation, damp-proofing and fire safety, which means an escape window and mains-linked smoke alarms. Adding an en-suite or wet room makes it far more useful and valuable.
Can I convert only part of my garage?
Yes. A partial conversion keeps the front section for storage or bikes and converts the rear into a utility room, WC or small office. It costs £10,000–£16,000 and needs the same Building Regulations sign-off as a full conversion.
Which areas does GSM Builders Ltd cover for garage conversions?
GSM Builders Ltd converts garages across West London from its base in Hayes UB3 – including Hillingdon, Uxbridge, Ruislip, Southall, Greenford, Northolt, Ealing, Acton, Hounslow, Harrow, West Drayton, Slough and Windsor. Call 020 8077 2121 for a free site visit.
Ready to turn your garage into a room you actually use?
Send GSM Builders Ltd a photo of your garage and tell us what you would like it to become. We will visit, measure up, check the planning position for your address and give you a free written quote.
