Architects for house extensions

What does an architect do on a house extension?

An extension architect is a person on the Architects Register who designs an addition to an existing home, prepares the drawings that councils and builders work from, and can guide you through the approvals. On a typical project the work falls into four jobs.

• Design. Turning a brief and a budget into a layout, a roof form and a choice of materials that suits the existing house and garden.
• Drawings. Survey drawings of what is there, then proposed plans, elevations and sections.
• Approvals. Working out which planning route fits and preparing the application or notice.
• Procurement and site. Per the Architects Registration Board, an architect can also obtain quotes from other construction professionals and manage the relationship between you and your builder (ARB checklist).

You can hire an architect for only the first three, or for all four. That choice moves the fee more than anything else, which is why it comes up again in the fees section below.

Which planning route will a rear or side extension take?

Which route a design takes is largely decided by two things: how far it projects and whether the land is designated. The table sets out the four routes for a single-storey rear extension on a house in England.

The limits themselves, and the conditions about eaves, the half-the-land rule and designated land, are set out in our guide to planning permission for a house extension. This page does not repeat them. What an architect adds is judgement at the edges. A scheme that is half a metre over the depth limit can sometimes be reshaped to fit, or deliberately taken down the prior approval route, and a design is only as good as the check made against the original house, which the Planning Portal defines as the house as first built or as it stood on 1 July 1948 (Planning Portal).

What does prior approval add for a larger single-storey extension?

Prior approval is a lighter check than a full application. Under the Order, adjoining owners and occupiers are given notice and their comments are to be received not less than 21 days from the date of that notice. If a neighbour objects, the council goes on to decide whether the effect on neighbouring amenity is acceptable (legislation.gov.uk). The Planning Portal describes the council as consulting adjoining neighbours about the proposal (Planning Portal).

For you, the practical point is time. The notice period and any amenity decision sit in front of the start date, so a project that needs this route should not have a builder booked for the week after submission. An architect who has run these applications will usually have views on how neighbours react to a deep extension and can show the council its effect on light and outlook in the drawings.

Do you need building regulations approval for an extension?

Yes, in most cases, and it is a different approval from planning. A scheme can be exempt from a planning application and still need full technical drawings for building control. On the full plans route the Planning Portal says the council must decide within five weeks, or two months if you agree to the longer period (Planning Portal).

The technical package is where an extension is really specified: foundations, the beam over a new opening in the old rear wall, insulation, drainage and ventilation. Ask each architect who will arrange the structural engineer's calculations and whether their fee includes them, because the answer is often "no" and the cost is separate. The drawings produced at this stage are also what builders price, so loose drawings produce loose quotes.

When does the Party Wall etc. Act come into an extension?

If the extension touches a shared wall, or digs near a neighbour's foundations, the Act can apply on top of planning and building regulations. The GOV.UK booklet gives two months' notice before work to an existing party wall, and one month before excavation within 3 or 6 metres, depending on the depth (GOV.UK). A side-return extension on a terrace is the usual example. Ask an architect whether the design needs notices, and see our page on party wall services for the surveyor side.

How do the RIBA stages apply to an extension?

RIBA, the architects' professional body, describes an architect's work in stages and says invoices may be monthly or sent at the end of stages. Its homeowner guidance groups them as stages 0 to 3 (feasibility studies, concept design and planning), stage 4 (technical design) and stages 5 and 6 (construction to handover) (RIBA). For a house extension that translates roughly as follows.

1. Stages 0 to 3: a measured survey, two or three layout options, a chosen scheme and the planning route, ending with a submitted application or notice.
2. Stage 4: the building regulations drawings, coordination with the structural engineer and a specification a builder can price.
3. Stages 5 and 6: tender and appointment of a contractor, visits during the build and a handover snagging list. Not every client buys this part.

How are architect fees structured for an extension?

RIBA lists three ways to set a fee: a percentage of construction cost, a lump sum, or a time charge, and says they can be mixed by stage (RIBA). Our guide to how architect fees work goes through each. For an extension the useful question is what the number covers. Check whether it includes the survey, the planning application, the building regulations package, the engineer, and visits on site. Council fees (£548 for a householder application, £249 for prior approval) are paid to the council and are separate from what the architect charges (Planning Portal). We do not quote typical fees here because no primary source publishes one for this kind of job. Get written quotes on the same brief and compare like with like.

What should you ask an architect before hiring them?

• Which approval route do you expect my extension to take, and why?
• Does your fee include the survey, the planning application and the building regulations drawings?
• Who arranges and pays for the structural engineer?
• Will you tell me if a party wall notice is likely to be needed?
• Do you offer the build stage, and what would it cost on top?
• What happens to the fee if the design changes after I have approved it?
• Can I see two similar extensions you have designed, and speak to a past client?
• Do you hold adequate professional indemnity insurance? The ARB checklist says your terms of engagement should confirm adequate cover.
• Will the person I meet be the person doing the work?

How do you check your extension architect is registered?

1. Ask for the architect's ARB registration number. The Architects Registration Board says every architect has a unique number and can give it to you on request.
2. Go to arb.org.uk and search the Architects Register by number, name or registered business address.
3. If you cannot find the person using the details they gave you, the ARB's guidance is that they are not an architect.
4. Repeat the check for anyone else who will be working on your drawings.

ARB says it is a criminal offence to use the title in business or practice without being listed (ARB). When you are ready to compare practices, you can browse architects by city, read the guides for background, or look at the related page on planning applications. Adding a floor rather than a room? See loft conversions.