Garage conversion vs extension: which costs less?
Converting a garage is far cheaper than building an extension: typically about £14,000, against roughly £40,000 to £64,000 for a 20 m² single-storey extension before fit-out. The garage route reuses an existing footprint, while an extension adds new space and uses garden area. Compare drawings and the same finish; neither project guarantees a higher resale return.
Garage conversion vs House extension: the cost
| Option | Range | Typical | Priced |
|---|---|---|---|
| Garage conversion | £7,500 to £20,000 | £14,000 | for the conversion |
| House extension | £2,000 to £3,200 | £2,500 | per m² |
Costwise guide prices, reviewed June 2026, not quotes. Note the two are quoted on different bases, so the figures above are not a like-for-like comparison on their own.
Garage conversion vs House extension: the difference
| Factor | How they compare |
|---|---|
| Typical cost | Garage conversion about £14,000; extension about £2,000 to £3,200 per m² (a 20 m² build is about £40,000 to £64,000). |
| Space and effort | A garage conversion is mostly internal and quicker. An extension is a bigger build but adds new, larger living space. |
| Trade-offs | Converting loses enclosed garage parking or storage. An extension takes garden space and costs much more. |
| Best for | Garage: the cheapest, fastest way to add a room. Extension: maximum new space and added value. |
Use the same usable room as the brief
Measure the inside of the garage after allowing for the proposed wall and floor build-up. Decide whether that space suits the intended office, bedroom or living room before comparing it with an extension. A conversion uses an existing footprint; it cannot gain width by removing the word garage from the plan.
The £14,000 garage figure is a typical project allowance rather than a fixed 20 m² price. For comparison, the extension base rate gives £40,000 to £64,000 at 20 m², before separate fees and fit-out. Get both options priced to drawings showing the same usable room and finish.
The garage condition can change the work
Ask the surveyor or builder to assess the floor level, damp protection, roof condition, insulation, ventilation and the former vehicle opening. Agree whether heating, electrical work and any drainage changes are included. Existing walls and a roof help only if they can be used in the finished design.
For the extension, compare foundations, drainage diversions, structural openings, glazing and making good. Ask each firm to price the work it has actually inspected, and identify provisional items rather than relying on a single per-m² allowance.
Account for the space you give up
A conversion removes enclosed parking or storage, although it does not necessarily remove the driveway parking outside. Price any replacement storage you need and check whether a parking-related planning condition affects the proposal. An extension uses garden space instead.
Choose the route that creates the right room at a cost you can afford. A larger project is not automatically the better investment: ask a local agent about the specific layout and the effect of losing the garage.
Common questions
Converting a garage is much cheaper, typically around £14,000, versus £40,000 or more for a single-storey extension of similar size, because the garage already has walls, a roof and a floor.
That depends on the proposed room and the local market. A conversion can create useful space at lower cost, but losing garage storage or enclosed parking may matter to buyers. Ask for a valuation of both proposed layouts rather than assuming a fixed return.
These are independent guide prices based on typical UK jobs in 2026. Your actual cost depends on your property, spec, access and where you live. Always get at least three written quotes before committing.