Extension Costing, know what your project should cost before you commit
Before you appoint a builder, you should know what the work should cost. Costed prepares independent pre-build budgets for extensions, loft conversions, basements and full renovations, then reviews contractor quotes line by line. The single best protection against overruns is knowing the number before the first quote lands.
- Itemised pre-build budget based on your drawings and current London rates.
- Contractor quotes compared line by line, anomalies flagged.
- Optional monthly cost monitoring during construction.
- Independent of every builder you might appoint.
| Project type | Typical cost per m² (London) | Notes |
|---|---|---|
| Single-storey rear extension | £3,200 – £4,500 | Excludes kitchen / fit-out |
| Double-storey rear extension | £2,900 – £4,200 | Per m² of new floor area |
| Loft conversion (dormer) | £2,800 – £4,000 | Inc. structure, en-suite |
| Basement (under existing footprint) | £5,500 – £8,500 | Highly site-dependent |
| Full house renovation (refurb) | £2,400 – £3,800 | Existing footprint, mid-spec |
Why homeowners get blindsided on cost
Most homeowners never see a real budget before quote. Their architect's ballpark is often optimistic; the first contractor's quote is often incomplete. By the time the third invoice lands, the gap between expectation and reality is already a five-figure problem. An independent pre-build budget closes that gap before it opens.
Frequently asked
How much does a cost expert cost for a domestic project?
For a typical London extension or renovation in the £150k–£500k range, our fixed-fee budget and quote review service usually sits between £1,200 and £3,500 depending on scope. Ongoing monthly monitoring is quoted separately.
Do I need a cost expert for an extension?
Legally, no. Financially, almost always yes if the works exceed roughly £100k. The fee is small relative to the variation and overrun risk it eliminates, and it puts you on a level footing with builders who price these projects every week.
How much does an extension cost per m² in London in 2026?
For 2026, single-storey rear extensions in London typically cost £3,200–£4,500 per m² excluding kitchen and fit-out. Double-storey extensions and loft conversions run slightly lower per m² thanks to shared structure. These are budget bands, every site is different, which is exactly why an itemised budget beats a rule of thumb.
Know what your project should cost. Before you build it.
One UK working day response. Fixed-fee proposal, VAT inclusive. No retainer. No obligation.