Who do I need for a house extension: architect, designer, engineer or builder?
4 min
9
September 2026. Costs are indicative 2026 UK ranges, not a quotation.
Key figures in this guide
One-minute decision
What are you trying to work out?
Use the checker first, then turn the result into a clearer brief before speaking to professionals.
Open the checkerA typical UK house extension needs four people: a designer to settle the layout and get planning, a structural engineer to design the beams and foundations, building control to inspect and certify, and a builder to construct it. A party wall surveyor joins if you build on or near a shared boundary.
The order in which you appoint them quietly determines your budget, your programme and how much control you keep. For most extension and renovation projects in the UK, design comes before price.
Who you need, and what each costs in 2026
| Who | What they do | Typical 2026 fee | Always needed? |
|---|---|---|---|
| Architect or architectural designer | Layout, drawings, planning application | £2,200 - £18,000 | Yes, someone must produce drawings |
| Structural engineer | Foundations, beams, roof calculations | £900 - £2,600 | Yes, for any extension |
| Building control | Plan check, site inspections, completion certificate | £700 - £1,900 | Yes, by law |
| Builder or main contractor | Builds it, coordinates trades | The build cost | Yes |
| Party wall surveyor | Notices and award with neighbours | £1,200 - £2,500 per neighbour | Only near a shared boundary |
| Planning consultant | Difficult, contested or refused applications | £900 - £3,500 | Occasionally |
| Energy assessor | SAP or glazing calculations | £250 - £600 | If glazing exceeds the allowance |
The usual order
- 01Designer — architect, architectural technologist or architectural designer. They test what is possible, produce drawings and secure planning.
- 02Structural engineer — appointed at technical design, sizes beams and foundations for building control.
- 03Specialist consultants as needed — party wall surveyor, principal designer for CDM, energy assessor, arboriculturalist if trees are involved.
- 04Contractor — appointed on completed drawings and a written specification, through a short tender.
Appointing a builder first is only sensible for genuinely simple, like-for-like work with no design decisions and no planning involvement.
Architect or architectural designer?
"Architect" is a protected title; only ARB-registered professionals can use it. RIBA membership is additional. Architectural technologists (CIAT) are strong on technical detailing and building regulations. Unregistered designers can be excellent and cheaper, but you have less recourse and less design ambition for difficult sites.
Choose an architect when the site is constrained, listed or in a conservation area, the design is ambitious, or the value at stake justifies the fee. Choose a technologist or designer for straightforward extensions where the shape is largely determined.
What design fees look like
Percentage fees for full service typically run 8-12% of build cost on domestic work, or 5-7% for planning-and-building-regs only. Stage-based fixed fees are common and easier to control. Whichever you choose, get the scope in writing: which RIBA stages, how many design revisions, whether the planning application, building regulations package, tender pack and site inspections are included. Site inspections are the item most often quietly excluded.
What to ask before appointing
- Show me three completed projects like mine, in this area.
- What is your planning approval rate with this council?
- Who will actually do the work — you or a junior?
- What is excluded from your fee?
- What do you think this project should cost to build? A designer who will not discuss budget is a risk.
- Are you covered by professional indemnity insurance, and at what level?
Design and build
A design-and-build contractor handles both. It is faster and gives one point of responsibility, but the designer works for the contractor, not for you, so value engineering happens in their interest. It suits standard extensions where speed matters more than design control. If you go this way, appoint your own independent adviser to review what is proposed.
Protect yourself on appointment
Use a written appointment — RIBA and CIAT publish domestic versions. Confirm copyright and licence to use the drawings if you part ways. Agree how fees are paid against stages, and never pay a full stage in advance.
The order that costs money
Getting builders to quote from sketches, then commissioning drawings that do not match, then re-pricing. You pay twice and lose two months. Fix the design, then buy the build.
Next step
Renovome Workspace matches you with designers, engineers and contractors near you from a network of over 1,000 UK practices, filtered by location and project type, and keeps the whole appointment trail in one place.
Last reviewed: September 2026. Costs are indicative 2026 UK ranges, not a quotation.