Loft conversion vs extension: which is cheaper?
A loft conversion is typically £45,000, while a 20 m² extension build is £40,000 to £64,000 before fit-out and separate fees. These are different budget scopes: compare the usable room, finish and access you need before deciding which is cheaper.
Loft conversion vs House extension: the cost
| Option | Range | Typical | Priced |
|---|---|---|---|
| Loft conversion | £22,000 to £75,000 | £45,000 | for the project |
| 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.
Loft conversion vs House extension: the difference
| Factor | How they compare |
|---|---|
| Typical cost | Loft conversion about £45,000 for the project; single-storey extension about £2,000 to £3,200 per m² (so a 20 m² build is about £40,000 to £64,000). |
| Space gained | Loft: a bedroom, often with an en-suite, upstairs. Extension: ground-floor living space such as a kitchen or family room. |
| Garden | A loft conversion keeps your garden; a ground-floor extension takes some of it. |
| Best for | Loft: an extra bedroom at lower cost without losing outdoor space. Extension: bigger ground-floor living or kitchen space. |
Compare the room you need, not just the headline price
A bedroom upstairs and a larger kitchen downstairs solve different problems. Start with the use, then compare usable floor area after stairs, circulation and low headroom. A nominal 20 m² loft floor can leave less usable space than a 20 m² rectangular extension.
The extension guide’s £2,000 to £3,200 per m² gives £40,000 to £64,000 for 20 m² before a kitchen fit-out and separate fees. The £45,000 typical loft figure is a whole-project allowance with no fixed floor area attached. It is not a matched 20 m² quote, so those figures alone do not prove which option is cheaper.
Ask for two drawings and the same finish
For each design, ask for a measured plan showing the usable room, door positions and furniture. For a loft, include the staircase and the space it takes from the floor below; for an extension, include the loss of garden and the new opening into the house.
Have both quotes list structural work, insulation, heating, electrics, internal finishes, design and approval fees. Add a bathroom or kitchen only to the option that actually needs one, and make that difference explicit. Compare the same finish rather than a loft shell with a fully fitted extension.
Disruption and resale need a property-specific answer
Consider where you can live and work while the job happens. A loft may involve opening the roof and forming a stairwell; an extension can interrupt a kitchen and access to the garden. Ask the builder to explain weather protection and when existing rooms become unavailable.
Choose the loft if an upstairs room solves the problem and the layout is practical. Choose an extension if the main need is connected ground-floor living space. Get a local valuation of the actual proposed layout before relying on added resale value; neither project guarantees a profit.
Common questions
The typical loft allowance here is £45,000, while a 20 m² extension build is £40,000 to £64,000 before separate fees and fit-out. The loft figure has no fixed floor area, so it does not prove a saving for equivalent space. Ask for drawings and quotes for the room you need.
Both can add value. A loft that adds a double bedroom and en-suite often returns well, while a kitchen extension can transform how a home feels. It depends on your property and area, and is never guaranteed.
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.