A garage conversion is substantially cheaper per square metre, because the structure, roof and foundations already exist. The work is insulating the floor, walls and roof to current standards, replacing the door opening with a wall and window, and dealing with damp and levels. The real cost is losing the storage, which people underestimate.
- You are buying insulation, damp-proofing and a new front wall, not a new structure.
- The floor is usually the problem: often below the house level and rarely insulated.
- Converting an integral garage is simpler than a detached one, which needs services run to it.
- Losing the storage is the regret people report most, not the room itself.
Why conversion is cheaper
The expensive parts of an extension are the ones a garage already has: foundations, external walls, and a roof. Converting means you are buying the upgrade rather than the structure.
That makes the per-square-metre figure dramatically lower than new build, and it is why a garage conversion is one of the better-value ways to add a room.
What it is not is cheap or trivial. A garage was built to keep a car dry, not to meet the thermal, damp and ventilation standards a habitable room requires. The work is real, and underestimating it is the commonest reason a conversion disappoints.
What the work actually involves
- Assess the existing structure. Foundations under a garage are sometimes shallower than a house, and the front wall will need building where the door was.
- The floor. Usually a concrete slab, often laid to fall towards the door, frequently with no damp-proof membrane and no insulation, and commonly a step below the house floor level. This is the biggest single item.
- Damp-proofing. Linking a new damp-proof membrane in the floor to the damp-proof course in the walls so there is a continuous barrier.
- Insulation to the floor, walls and roof, to current standards. On a single-skin garage wall this means an internal lining, which costs you floor area.
- The door opening. Build a new wall, with a window, on a foundation adequate to carry it.
- Services. Electrics, heating and sometimes plumbing, extended from the house.
- Ventilation appropriate to the room's intended use.
- Building Control notification, inspections and completion certificate.
Step two is where budgets move. A floor that needs digging out, a new membrane, insulation and a screed to bring it level with the house is a substantial job in itself.
Conversion against extension
| Garage conversion | Extension | |
|---|---|---|
| Cost per m² | Substantially lower | Higher |
| Area gained | Fixed — the garage footprint | Whatever you design |
| Garden lost | None | Real |
| Planning | Usually permitted development | Often, but less reliably |
| Building Regulations | Yes | Yes |
| Time on site | Weeks | Months |
| Disruption | Contained — often sealed off | Substantial |
| Storage lost | All of it | None |
| Parking affected | Possibly | No |
| Flexibility of layout | Limited by existing walls | Design freedom |
The honest summary: conversion wins on cost, speed and disruption; extension wins on size, flexibility and keeping the storage. They are not really competing for the same job.
The storage problem
This is the thing people report regretting, and it is rarely in the decision when it should be.
A garage holds the bikes, the mower, the tools, the paint, the Christmas decorations, the suitcases, the garden furniture over winter and the boxes nobody has opened since moving in. Convert it and all of that needs somewhere else to be.
In practice it usually ends up in the loft, in a new garden shed, or spread through the house. The first is fine if the loft is accessible and not earmarked for a future conversion. The second is a cost worth putting in the budget, and it uses garden. The third is how a house that gained a room ends up feeling no larger.
The practical advice: before converting, genuinely work out where the contents will go, and price the shed if that is the answer. It is a small addition to the budget and it is the difference between gaining a room and relocating a problem.
Integral, attached or detached
An integral garage — within the main footprint, usually with an internal door — is the easiest and best-value conversion. Services are adjacent, the structure is part of the house, and the new room connects naturally. The wall between the garage and the house, and the ceiling if there is a room above, may already have fire-resisting construction, which helps.
An attached garage to the side is similar, though it may need a new internal opening formed, which is structural work.
A detached garage is a different proposition. Converting it gives you an outbuilding rather than a bigger house, and services have to be run to it — a new electrical circuit in armoured cable, and heating and plumbing if it is to be more than a studio. It works well as a garden office or a gym, and it does not solve a shortage of space in the house itself.
Planning, parking and the practicalities
- Planning is usually not needed for converting a garage within the existing footprint, since it is an internal alteration and there is no extension. An Article 4 direction can change that, and some estates have conditions requiring the parking to be retained.
- Parking. Check whether any planning condition on the original development requires a parking space. It is not unusual, particularly on newer estates, and it can make a conversion unauthorised.
- Building Regulations definitely apply — thermal performance, damp, ventilation, structure for the new front wall, and fire safety.
- The driveway. Once the door is a wall, you may still park in front of it. Worth thinking about how that looks from the street and whether the new window faces a parked car.
- Future buyers. Some want a garage. Converting one in an area where most retain theirs can narrow the pool slightly, though it rarely outweighs the gain of a usable room.
What it costs
These are the figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Build cost | £1,500–£2,500 per m² | Spec dependent |
| Single-storey extension | £20,000–£60,000 | Size & finish dependent |
| Average project | ~£30,700 | Across typical jobs |
A conversion sits well below the new-build extension rate because the structure exists. The variables are the floor — whether it needs digging out, insulating and re-levelling — and how much of the wall construction needs lining to meet current standards.
We will tell you what the floor and walls actually need, and flag any parking condition before you commit.
Frequently asked questions
Is a garage conversion cheaper than an extension?
Substantially, per square metre, because the foundations, walls and roof already exist — you are buying the upgrade rather than the structure. It is not trivial work though: the floor usually needs digging out, insulating and re-levelling, the walls need lining, and a new front wall and window replace the door.
Do I need planning permission to convert a garage?
Usually not, where the conversion is within the existing footprint, because it is an internal alteration rather than an extension. An Article 4 direction can change that, and some estates carry a planning condition requiring the parking space to be retained — which is worth checking before committing.
What is the hardest part of a garage conversion?
The floor. A garage slab is typically laid to fall towards the door, often has no damp-proof membrane and no insulation, and usually sits below the house floor level. Digging out, laying a membrane, insulating and screeding to match the house is the biggest single item in most conversions.
Can I convert a detached garage?
Yes, and the result is an outbuilding rather than a bigger house. It works well as a garden office, gym or studio. Services have to be run to it — a new circuit in armoured cable, plus heating and plumbing if needed — and it will not solve a shortage of space inside the house itself.
Will converting my garage reduce my house value?
Rarely, provided the work is done properly and the storage problem is solved. A well-finished extra room is usually worth more than a garage. It can narrow the buyer pool slightly in areas where most houses retain a garage, and a poorly finished conversion that is obviously a converted garage helps nobody.
Where does everything in my garage go?
This is the question to answer before starting, and the one people most often skip. Realistically it goes to the loft, to a new garden shed, or into the house. Price the shed if that is the plan — it is a small addition to the budget and it is the difference between gaining a room and relocating a problem.
Check the floor before you budget
The floor and any parking condition are the two things that decide whether a garage conversion is straightforward. We check both at survey. Free quote.