The short answer
For a two-storey extension, an architect's fee is usually either a fixed fee for design and drawings — sitting higher than for a single-storey extension because there's more design and structure to resolve — or a percentage of the construction cost (commonly around 7%–15%) for a full service that includes overseeing the build. A two-storey extension affects the house's elevations, roofline and structure on two levels, and is more likely to need full planning permission than a single-storey one, so the architect's role is larger. The fee is separate from the build cost, the structural engineer, the planning fee, Building Control and any Party Wall surveyor. Because the design has to work over two floors and tie into the existing house convincingly, this is a project where an architect's input tends to be worth the fee.
Adding two floors at once is a bigger design and structural job than a single-storey extension, and the fee reflects that. Here's what to expect and why it sits higher.
Architect cost — two-storey extension
- Design + drawingsFixed fee (higher than single-storey)
- Full service~7%–15% of build cost
- PlanningMore likely needed
- Separate costsEngineer, council fees, Party Wall
- DriverTwo floors + elevations
Why it costs more than a single-storey extension
A two-storey extension is more involved than a single-storey one in several ways, each of which adds to the design and drawing work:
- Two floors of layout: the design has to work downstairs (often kitchen or living space) and upstairs (often a bedroom or bathroom), and connect to the existing rooms on both levels.
- Elevations and roofline: it changes how the house looks from outside on two storeys, so the design has to tie into the existing elevations, windows and roof convincingly.
- More structure: two floors and a roof mean more load and more structural design — additional beams and foundations — which feeds into the building-regs drawings and the engineer's work.
- Planning: two-storey extensions more often need full planning permission rather than falling under permitted development, so there's a planning process to manage.
All of this means more of the architect's time and skill, which is why the fee sits above a comparable single-storey extension.
Typical fees and separate costs
The architect's fee is one part of the overall budget; the build dominates, and several other fees sit alongside.
| Item | Indicative figure | Notes |
|---|---|---|
| Architect — design + drawings | Mid four figures+ | more than single-storey |
| Architect — full service | ~7%–15% of build | if overseeing the build |
| Structural engineer | Separate fee | beams, foundations, two floors |
| Planning fee | Paid to council | often full permission |
| Building Control + Party Wall | Separate | build checks, neighbour boundary |
Indicative UK figures for guidance only. Excludes the build cost and VAT. Source: Checkatrade and HomeOwners Alliance cost guides.
When an architect earns the fee here
Two-storey extensions are one of the projects where an architect's design skill most clearly pays off, because the result is so visible and permanent. A poorly designed two-storey extension can unbalance a house's proportions, clash with the existing elevations, or create awkward internal junctions where new meets old on two levels. A well-designed one looks as though it was always part of the house and makes both floors work harder. That's a design problem, and it's exactly what an architect is trained to solve.
The planning dimension adds to the case. Because two-storey extensions more often need full planning permission, and because they're more visible to neighbours, getting the design and the application right first time matters — a refusal costs time and money. An architect who knows the local planning context can shape a scheme that's more likely to succeed. As always, be clear which stages the fee covers (design and planning only, or overseeing the build too), and budget the whole project — architect, engineer, statutory fees, Party Wall and the build — rather than the design fee alone. On a project of this scale, the design quality and the planning success the architect brings usually justify the higher fee.
Planning and neighbour considerations
A two-storey extension is more likely than a single-storey one to raise planning and neighbour issues, and these are worth understanding early because they affect both the design and the timetable:
- Full planning permission: two-storey extensions more often fall outside permitted development, so you'll usually need to apply for full planning permission. That brings a formal application, a consultation period and a decision timescale to plan around.
- Overlooking and overshadowing: adding height near a boundary can raise concerns about overlooking neighbours' windows or gardens, or casting shade. Planners weigh these, and a thoughtful design that sets back or angles the extension can address them.
- Matching the house: because the extension is visible on two storeys, planners often expect it to respect the existing house's proportions, materials and roofline. A design that ties in convincingly is more likely to be approved.
- Party Wall: work near or on a shared boundary usually engages the Party Wall process, which means serving notice on affected neighbours and possibly appointing a surveyor — a separate process and cost that also takes time.
- Conservation areas and special designations: these add further constraints and make professional design input more valuable.
Handling these well is a large part of what an architect contributes on a two-storey extension. Getting the design right for the planning context, and managing the neighbour and Party Wall steps in good order, reduces the risk of a refusal or a dispute that delays the whole project. Factor the planning and Party Wall timescales into your plans from the start, because they can add weeks or months before any building work begins — something homeowners focused on the build cost alone often underestimate.
Frequently asked questions
Why does a two-storey extension cost more in architect fees than a single-storey one?
Because there's more work: two floors of layout, changes to the elevations and roofline on two storeys, more structural design, and a greater likelihood of needing full planning permission. All of that means more of the architect's time and skill, so the fee sits higher.
Does a two-storey extension need full planning permission?
More often than a single-storey extension, yes. Two-storey extensions are more likely to fall outside permitted development, especially in conservation areas, on flats, or where they're close to boundaries. You'll also need building-regulations approval regardless. Check with your local authority.
Will I need a Party Wall surveyor?
Often, for a two-storey extension near a shared boundary. The Party Wall process means notifying affected neighbours and may involve appointing a surveyor. It's a separate matter and cost from the architect's fee, though the architect can flag when it applies.
Sources & further reading
- Planning Portal — extensions
- GOV.UK — Party Wall etc. Act explained
- Checkatrade — two-storey extension cost guide
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.