The short answer
For a typical UK house extension, architect fees usually fall in the region of £1,500–£8,000+, depending on how much of the work you commission. Plans-only packages — concept design plus the planning and building-regulations drawings — commonly sit around £1,500–£5,000, while a full service that runs design through to overseeing the build is often charged as a percentage of build cost and lands higher. On a full-service basis the fee is typically around 10–17% of build cost for extensions, because a one-off extension needs more design time per pound than a large simple build. London, two-storey extensions, sloping or constrained sites, and listed or conservation-area properties all push the figure towards the top of the range.
Most homeowners on an extension commission the early design and drawing stages and manage the build themselves. What you pay depends mainly on how far through the RIBA stages you take the architect, and how complex your site is. The figures below are typical ranges for guidance, not quotations.
Typical extension figures
- Plans-only package~£1,500–£5,000
- Full service~10–17% of build cost
- Typical fee span~£1,500–£8,000+
- Pushes it higherLondon, two-storey, listed
- Most homeowners usedesign + drawings only
Plans only versus full service
- Plans only (RIBA 0–4, roughly): feasibility, concept design, the planning application drawings and the building-regulations package — commonly around £1,500–£5,000 for an extension. You then run the build with a builder or project manager.
- Full service (RIBA 0–7): everything above plus tendering, contract administration and site inspections through to handover. This is usually charged as a percentage of build cost and costs more, but the architect oversees the build for you.
| Service | Typical figure | What it covers |
|---|---|---|
| Plans only | ~£1,500–£5,000 | concept, planning & building-regs drawings |
| Full service (extension) | ~10–17% of build | design through to overseeing the build |
| Hourly add-ons | ~£50–£150 / hour | site visits, revisions, advice |
Indicative UK figures for guidance. Sources: HomeOwners Alliance and Architecture for London fee guidance.
What pushes the cost up
The same extension costs more to design where the work is harder: a two-storey or wrap-around extension involves more structure and detail than a single-storey rear; a sloping, tight or awkward-access site adds design time; and a listed building or conservation area brings extra drawings and consultation. London and the South East also sit higher on fees generally. If your scheme is straightforward, a plans-only package from an architect — or an architectural technologist — often covers it for less.
Planning an extension?
We'll match you with a RIBA-chartered architect who scopes your extension, explains plans-only versus full service, and quotes on a clear fee basis for your property.
Frequently asked questions
How much does an architect cost for a house extension?
Typically in the region of £1,500–£8,000+ depending on scope. Plans-only packages — concept, planning and building-regulations drawings — commonly sit around £1,500–£5,000, while a full service that oversees the build is usually a percentage of build cost (often around 10–17% for extensions) and costs more.
Do I need full service or just drawings for an extension?
Many homeowners commission the early RIBA stages — design and the planning and building-regulations drawings — and manage the build themselves, which keeps the architect's fee lower. A full service costs more but means the architect oversees the build through to handover.
Why do extension fees vary so much?
Because extensions vary. A single-storey rear extension on a simple site costs less to design than a two-storey, wrap-around or listed-building scheme. Location matters too, with London and the South East generally higher.
Sources & further reading
- HomeOwners Alliance — architect fees and how to cut them
- Architecture for London — architects' fees in the UK
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.