RenoCost

Architect Fees for an Extension UK

Updated for 2026 UK pricing

Planning a rear, side or wraparound extension and wondering what the architect will cost? UK architect fees for a home extension in 2026 typically range from around £2,500 for planning drawings only through to £8,000–£12,000+ for a full service that also covers Building Regulations drawings, tendering and construction-stage inspections. This guide breaks down what drives those numbers and what to budget.

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Key Takeaways

  • Planning drawings only: often £2,500–£4,500 for a single-storey extension.
  • Planning + Building Regs: typically £4,000–£7,500.
  • Full service (design → construction): commonly £6,500–£12,000+.
  • Add a structural engineer (£800–£2,500) and party wall surveyor if applicable.
  • London and the South East typically add 10%–25% to fees.

What UK architects charge for extensions

Extensions are one of the most common residential briefs, and most architects and technologists have priced scope for them. Broadly:

  • Small single-storey rear extension (10–20m²): planning drawings £2,500–£4,500; full service £6,500–£10,000.
  • Side-return or larger single-storey (20–40m²): planning £3,000–£5,500; full service £8,000–£12,000.
  • Wraparound or double-storey extension: planning £4,000–£7,000; full service £10,000–£18,000.

These are indicative ranges based on published industry guidance and typical UK residential practice. Real quotes will vary by architect, region and how well-defined your brief is. Use the Architect Fees Calculator to model your specific project.

What drives the price up or down

  • Location. London and parts of the South East run 10%–25% above the national average.
  • Complexity. Structural walls being removed, bespoke glazing, or unusual site levels all add work.
  • Planning context. Conservation areas, national parks, listed buildings and previous refusals push fees up.
  • Scope you buy. Planning-only is the smallest scope; full service with construction administration is the largest.
  • Existing information. Having a measured survey or existing drawings can reduce fees a little.

Planning drawings vs Building Regulations drawings for extensions

Planning drawings show the local authority what the extension will look like. They are not, by themselves, enough to build from. Building Regulations drawings add the technical detail your builder and Building Control need — insulation, structural sizing, fire compartmentation, drainage, glazing U-values. Most extensions need both. Some architects quote them as one package; others as two stages. Always confirm which you are getting.

Other professional costs on an extension

  • Structural engineer — sizing beams, foundations and structural openings (£800–£2,500).
  • Party wall surveyor — required where work affects a shared wall (£900–£2,200).
  • Drainage / topographical survey — where drains or ground levels are unknown (£300–£1,500).
  • Heritage statement — for listed or conservation-area properties (£500–£1,800).
  • Energy / SAP assessment — often needed for larger extensions (£150–£450).

Several of these items — plus revisions, disbursements and construction-stage support — are commonly left out of the headline architect quote. See our guide to hidden architect fees before you sign an appointment.

Statutory fees paid to the council

Planning application fees vary by proposal type and UK nation. Check the current fee with your local planning authority before setting your budget. Building Control fees are typically £500–£1,500 depending on the extension. These are paid to the council or approved inspector, not to your architect. Current application fees are published on the Planning Portal and via GOV.UK.

In the UK, the title "architect" is protected by law. Check that your architect is listed on the Architects Register before appointing them.

How to compare architect quotes for an extension

  1. Ask each professional to quote by RIBA stage (or equivalent) so you can compare like-for-like.
  2. Check whether Building Regulations drawings and the specification are included.
  3. Confirm VAT status and whether the fee shown is inclusive.
  4. Ask about the hourly rate for variations and how many design options are included.
  5. Compare portfolios of similar-sized extensions in similar property types.

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How We Calculate Costs

RenoCost estimates are calculated from published UK cost benchmarks and RenoCost's documented per-module cost assumptions. The model adjusts these assumptions using the property characteristics, selected renovation scope, finish level and broad postcode-based pricing region, then presents a low, mid and high planning range.

  • Published UK benchmarks — macro reference points for construction costs and materials and labour trends, used to sense-check RenoCost's own assumptions.
  • RenoCost's per-module assumptions — cost and duration assumptions for each renovation module, reviewed and updated periodically by the RenoCost team.
  • Estimate ranges — low, mid and high bands set to reflect the realistic spread between competitive and premium specifications, rather than a single fixed price.

Data confidence note

Completed calculator estimates are used to produce anonymous renovation-planning insights. They are not contractor quotations, invoices or confirmed final project costs. RenoCost estimates are intended for initial planning and budgeting. Property condition, specification, access, contractor availability and site-specific requirements can change the final cost. Always obtain detailed written quotations before committing to a project.

Regional adjustments are applied based on postcode area to reflect broad differences in local labour and material rates. See our Method Glossary for definitions of the terms used here.

Related Guides

Architect Fees Calculator UK →Hidden architect and professional fees →

Frequently Asked Questions

How much do architect fees cost for a UK extension?

For a typical single-storey rear or side extension, UK architect or designer fees generally range from around £2,500 for planning drawings only to £8,000–£12,000+ for a full service including Building Regulations drawings and construction-stage support. Fees vary significantly by region, complexity and how much of the process you want the architect to lead.

What percentage of the extension build cost is the architect fee?

A common industry range for a full architectural service on a residential extension is roughly 7%–15% of construction cost, with smaller and more complex projects sitting at the higher end. For planning drawings only, expect a much smaller share — often a fixed fee that works out at 3%–5% of build cost or less.

Do I need an architect for a small rear extension?

Legally, no — many extensions built under Permitted Development or straightforward planning applications are designed by architectural technologists or residential designers. Practically, an experienced designer or architect will usually pay for themselves in avoided mistakes, better layouts and smoother planning approval.

Are Building Regulations drawings included in an extension architect fee?

Not always. Some architects quote for planning-stage only and price Building Regulations drawings as a separate stage. Always confirm whether Building Regulations drawings are included; without them, your builder cannot properly price or build the extension.

What extra professional costs come with an extension?

On most extensions you will also need a structural engineer (£800–£2,500), a party wall surveyor if you share a wall with a neighbour (£900–£2,200), and possibly a drainage survey or heritage statement in conservation areas. Statutory fees (planning, Building Control) are paid separately.