The short answer
For a single-storey kitchen extension, planning drawings typically cost £1,000 to £2,500. A full service, from design through tender to inspecting the work on site, is usually charged at 3% to 15% of the build cost. The structural engineer, planning fee and building control are separate bills. These are typical national ranges for 2026, including VAT.
What the fee pays for
Design work on an extension comes in stages, and most designers will quote for each one separately if you ask. That matters, because you may not need all of them.
Measured survey. Someone measures the house and draws it as it stands. Every later drawing depends on this one being right.
Design and planning drawings. Plans and elevations of the proposal, good enough for a planning application or a lawful development certificate. This is the stage where the room gets its shape, so it is where a good designer earns the fee.
Building regulations drawings. A far more detailed set: foundations, wall and roof build-ups, insulation, drainage, ventilation, fire safety. Building control approves these, and builders price from them. A job priced from planning drawings alone is a job priced by guesswork.
Tender and contract. The designer sends the drawings to builders, compares the prices and prepares a written contract.
Site inspections. The designer visits during the build, answers the builder’s questions and certifies each stage payment.
Planning drawings for a single-storey extension, with the survey they rest on, typically cost £1,000 to £2,500. Building regulations drawings are often a further fee. Many homeowners stop there and manage the builder themselves. The last two stages keep the designer involved for months longer; a full service is usually charged at 3% to 15% of the build cost, or as a further lump sum. These are typical national ranges for 2026, including VAT, and London and the South East usually cost 20 to 40% more.
Architect, technologist or designer
“Architect” is a protected title. Only someone on the Architects Registration Board’s register may use it, and you can look any name up there in a minute. An architect has completed a long accredited training and must carry professional indemnity insurance.
Many extensions are drawn by architectural technologists or by unregistered designers, often at lower fees. Plenty are very good at exactly this kind of work. The checks are the same whoever you hire: ask to see two or three built extensions like yours, ask what insurance they carry, and get the fee and the list of drawings in writing.
For a plain rectangular extension under permitted development, a technologist’s drawings are usually all you need. For a listed building, a conservation area or a tight site with unhappy neighbours, an architect who knows the local planning officers is money well spent.
The bills that are not the architect’s
Homeowners often expect the design fee to cover everything on paper. It does not. Allow for these as well:
| Item | Typical cost | Who you pay |
|---|---|---|
| Structural engineer | £600 to £1,200 single storey, £800 to £1,800 two storeys | The engineer, for beam and foundation calculations |
| Building control | About £700 to £1,100 for a single storey of 10 to 40 m² | The council or a registered approver; each sets its own fees |
| Householder planning application | £548 | The council, in England |
| Lawful development certificate | £274 | The council, in England |
| Party wall surveyor | £700 to £1,500 per neighbour for one agreed surveyor; more if each side appoints its own | Only if a neighbour does not consent; you pay both surveyors |
On a single-storey extension, planning drawings, engineering and building control together come to roughly £2,300 to £4,800, before any building regulations drawings. All professional fees together usually come to 10% to 15% of the build cost. Set against the building work that is a small share of the whole, and it is the share that decides how well the rest is spent.
Fixed fee or percentage?
RIBA notes that architects may charge a fixed fee, a percentage of the build cost or by the hour. A fixed fee suits a small, well-defined job and tells you where you stand. Ask what happens if the planners want changes: one round of revisions is normally included, a redesign is not.
A percentage fee rises with the build cost, which can feel like paying more for the designer’s own extravagance. It is more common where the designer runs the whole project through to completion. If you are offered one, ask for it to be capped or converted to a lump sum once the design is fixed.
Hourly rates are best kept for small extras, such as a site visit to settle a query.
Where to save, and where not to
You can save by having a clear brief before the first meeting: what the room is for, how many sit at the table, what you have seen and liked. Indecision is billed by the hour, one way or another.
Do not save on the building regulations drawings. A thin set leaves the builder to decide the details, and every detail decided on site is one you did not get competing prices for. A full set, sent to three builders, usually repays its fee in the spread between their quotes.
Fees are one line in the budget. For the rest, the price board and calculator on our kitchen extension cost guide add building work, the kitchen and fees into one range.