Ask five practices for a quote on the same extension and you’ll get five different numbers, sometimes across a double range. On a £90,000 build, fees can land anywhere between £7,200 and £13,500, and the gap usually has nothing to do with quality.
It comes down to what sits inside the percentage. A drawings only quote and a full service quote carry very different prices for what most homeowners assume is the same job. Comparing architect london on headline percentage alone is how people end up paying twice.
How Much Does an Architect in London Cost for a House Extension
Three service tiers explain most of the variation you’ll see. What separates them is how far past the drawings the practice takes you.
- Drawings only. Concept sketches and planning application drawings, nothing further
- Planning and building regulations. Drawings plus submission, technical package and structural coordination
- Full service. Design through to tender, contract administration and site visits
A full service practice sits higher on price than a freelancer producing drawings. That higher figure often removes the need to separately appoint a structural engineer, a planning consultant and a project manager. Chase the lowest headline number and those three roles usually reappear later as separate invoices.
The Percentage of Build Cost Pricing Model
Most UK practices price as a percentage of construction cost rather than a flat fee, because scope shifts once a survey reveals the real structural picture. A fixed fee doesn’t flex with that, which is why fixed quotes tend to come with tighter scope definitions.
| Service level | Typical fee, percentage of build cost | What’s included |
| Drawings only | 3 to 6 percent | Concept design, planning drawings |
| Planning and building regs | 5 to 10 percent | Above plus submission and technical drawings |
| Full architectural service | 8 to 15 percent | Above plus tender, contract administration, site visits |
| Complex or heritage projects | 13 to 17 percent | Full service on constrained or listed sites |
The percentage generally falls as project value rises, because the fixed cost of running a project doesn’t scale proportionally. A £60,000 extension and a £300,000 one need broadly the same volume of drawings and correspondence.
A quote at 8 percent and a quote at 13 percent are rarely cheap versus expensive. They’re usually two different scopes of work sitting under the same job title, and the only way to tell is to compare the RIBA stages each one covers.
What Affects Architect Fees for a Single Storey Extension
Property type and structural complexity move the figure more than anything else. Construction for a London single storey extension commonly runs £2,500 to £3,800 per square metre excluding VAT, so a 20 to 30 square metre rear extension sits at roughly £50,000 to £114,000 in construction alone.
That figure is construction only. Professional fees, planning costs and VAT sit on top of it.
A rear extension on a detached house involves far less negotiation than a side return on a mid terrace inside a conservation area. Listed status, tight boundaries and period construction all add hours at design and planning stage. Party wall agreements need drafting, boundary positions need confirming, and older properties usually need a more detailed structural survey before drawings start.
Planning adds its own line. The statutory determination target for a householder application is eight weeks, and that time is billed whether the application succeeds or not. A refused first submission, which happens regularly on constrained urban plots, brings resubmission costs with it.
The Two Costs Most Quotes Leave Out
Both are avoidable surprises, and both are large.
The council fee. A householder planning application in England now costs £548, rising by CPI every 1 April under the Levelling Up and Regeneration Act 2023. It was £258 until April 2025, so any budget or guide still quoting that figure understates it by more than half. The Planning Portal adds a service charge of around £91 on top if you submit through it.
VAT. Construction and professional fees on most extension work attract VAT at 20 percent. Quotes are frequently presented excluding it, which is legitimate but means a £90,000 build is a £108,000 payment. Confirm the VAT position in writing on every quote before comparing them.
In House Against Outsourced Structural Engineering
Where the calculations sit changes both cost and programme, not just the invoice count.
An outsourced engineer typically needs near final drawings before signing off calculations. A late design change then triggers a fresh round of fees and a fresh wait, because the drawings have to go back out and come back in. Practices with the engineer in the same team resolve those changes internally.
On cost, structural engineering for a domestic extension commonly runs from around £90 an hour on an ad hoc basis, with full calculations and drawings for a typical home extension in the region of £1,000 to £2,500. Whether that’s inside the architectural fee or outside it is one of the first questions to ask, because it’s the single most common reason two quotes aren’t comparable.
How to Use a Cost Calculator Before You Budget
Establish your construction budget before your first call, not after. It gives you a figure to test any fee quote against, rather than accepting the first percentage you’re offered.
The sequence:
- Enter your property type and approximate extension area, width multiplied by depth
- Get an estimated construction cost range
- Apply 5 to 15 percent depending on the service level you actually want
- Add the £548 application fee, structural engineering and VAT
- Compare that total against the quotes you receive
A single-story extension cost calculator will get you to step two in about a minute, though most of these tools exclude VAT and detailed interior specification, so read what’s covered before treating the output as a budget.
Worked example. A 20 square metre mid range extension at £3,000 to £3,800 per square metre is £60,000 to £76,000 in construction. Add professional fees at 10 to 15 percent and you’re at £66,000 to £87,400, then add the council fee and VAT to reach the actual cash requirement.
Step up to 30 square metres at the same specification and construction alone runs £90,000 to £114,000. At a 12 percent full service fee that’s roughly £10,800 to £13,700 in architectural, planning and structural coordination costs.
Choosing the Right Architect for Your Budget
Rank quotes by scope, then by price. A practice quoting a higher percentage while covering structural engineering, planning submission and project management under one roof frequently works out cheaper once every real cost is counted, because three separate consultants each look affordable alone and still total more once coordination time and delays are included.
Ask three questions of every quote. Which RIBA stages does it cover, is structural engineering inside or outside the fee, and is VAT included. Those three answers make otherwise incomparable numbers comparable, and they matter more than the percentage itself.
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