For homeowners planning an extension or loft
Architect for a House Extension: What to Expect
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 5 min read
The short answer
You do not need an architect by law for a house extension, but you do need drawings that the council and building control will accept. An architect or designer turns your brief into those drawings and, on a full service, follows the work to completion.
- Many rear and loft extensions fall under permitted development; conservation areas, flats and larger schemes need planning permission.
- Every extension needs Building Regulations approval, and work near a neighbour's wall can need Party Wall Act notices.
- Most extension drawings are priced per set or as a percentage; engineer, council and building control fees are extra.
- Only someone on the Architects Register may trade under the title architect, so check the register.
You do not need an architect by law to build a house extension. You do need drawings the council and building control will accept, and a design that works for the house and the budget. An architect or designer turns your brief into those drawings and, on a full service, follows the work to the end.
This guide covers what an extension architect draws, when you need planning permission, building regulations and party walls, and how fees are charged. It also covers loft and mansard extensions, the commonest way to add space to a London terrace.
Comparing residential architects in London? Our design team draws the extension and our own builders build it, in one company. Book a free design consultation and we will look at the house with you.
When you need an architect for an extension
There is no legal requirement to use an architect for an extension. Anyone can prepare the drawings: an architect, an architectural technologist, a designer or a builder's own design team.
What the law protects is the title. Under section 20 of the Architects Act 1997, using the title architect in business without being on the Architects Register is a criminal offence. If someone calls themselves an architect, check the register.
An experienced designer earns the fee on harder projects. A listed house, a conservation area, a basement, a two-storey side extension or a tight terrace with neighbours on both sides all reward careful drawing. A simple single-storey rear extension needs less.
What the architect draws for an extension
An extension needs two sets of drawings: one for consent and one to build from. On a full service the same person or team prepares both.
- Measured survey. The existing house, drawn accurately before anything is designed.
- Planning or permitted development drawings. Existing and proposed plans, elevations and sections, with a location plan and site plan.
- Building regulations drawings. Construction details for structure, insulation, fire safety, drainage and ventilation.
- Structural engineer's calculations. Beams, foundations and any steel, prepared by an engineer.
- Construction information. Specifications and details the builder prices and builds from.
The HomeOwners Alliance guide notes that an architect cannot do the technical calculations, so a structural engineer is always part of the team. Ask who appoints the engineer and who coordinates the two sets of drawings.
Planning permission vs permitted development
Many extensions do not need a planning application at all. Permitted development lets you extend a house within set limits, and the drawings simply have to prove the scheme stays inside them.
| Extension | Permitted development limit | Notes |
|---|---|---|
| Single-storey rear | 3 m (semi or terrace), 4 m (detached), 4 m high | Up to 6 m and 8 m through the larger home extension prior approval |
| Side | Single storey, no wider than half the original house | Not on article 2(3) land |
| Loft | 40 m³ (terrace), 50 m³ (other houses) | Not beyond the roof slope facing a highway |
| Any extension | Not more than half the land around the original house | Earlier extensions count |
Flats and maisonettes have no householder permitted development. Conservation areas, which are article 2(3) land, lose several of the rights above. In much of inner London that means a planning application. Our guide to permitted development extension rules sets out every limit.
If the extension is permitted development, a lawful development certificate under section 192 of the Town and Country Planning Act 1990 confirms it. It is optional, but useful when you sell. Where planning is needed, the householder application fee in England has been £548 since 1 April 2026.
Building regulations and the Party Wall Act
Every extension needs Building Regulations approval, whether or not it needs planning permission. GOV.UK says approval comes from your council's building control or a private registered building control approver. Without it, you will not have the certificates you may need when you sell.
The building regulations drawings show how the extension meets each part of the regulations: structure, fire, insulation, ventilation and drainage. Building control checks the drawings and inspects the work on site.
The Party Wall etc. Act 1996 is separate from both. You must serve notice on your neighbour before building on the boundary line, working on a party wall or digging near their foundations. Work to a party wall needs at least two months' notice, and excavation one month. Our guide to the Party Wall Act explains the process.
How extension architect fees are charged
Extension drawings are usually priced as a fixed fee for each set, or as a percentage of the build cost. The HomeOwners Alliance guide for 2026, citing Checkatrade, puts single-storey extension planning drawings at £1,000 to £2,500, and two-storey at £2,000 to £3,500.
The same guide puts building regulations and construction drawings at £3,000 to £5,000 for a single-storey extension, and £5,000 to £7,000 for a double-storey one. These are national figures; a London house with a basement or a listed facade costs more to draw.
On top of the drawing fee, budget for the structural engineer, building control, the council's planning fee where needed and any party wall surveyors. Our guide to architect fees in the UK covers the fee models stage by stage. We agree our own design fee per stage, in writing, once we have seen the house.
Loft and mansard extensions
A loft conversion is one of the commonest ways to add space to a London terrace. Under permitted development, a terraced house can add up to 40 m³ of roof space and other houses 50 m³. Nothing may go beyond the roof slope facing the street.
The HomeOwners Alliance guide quotes £900 to £1,500 for loft planning drawings, or £1,400 to £2,000 with building regulations drawings. A loft also has to meet the regulations on stairs and fire escape, and the drawings must show how.
A mansard reshapes the roof with steep sides and a flatter top. Councils judge it on the character of the building and the street. Kensington and Chelsea, for example, assesses roof additions against the form of the building and its group. Raising a party wall for a mansard also needs a party structure notice.
Our guide to the mansard roof covers the planning side in more depth.
I and my husband are very satisfied with Craftex's service and customer support. We bought a house in Hampstead and wanted to redesign and renovate it. On our initial free consultation with Sviat, we found out that we actually can do loft conversion and rear extension and add value to the property straight away. We worked with the Craftex design team to bring our vision to life and deliver planning approvals. At that time guys started stripping out to save time on the construction programme as we wanted to move in ASAP. We worked with very experienced project director Konstantin who was responsible for all aspects of project delivery from design to construction with a very good eye on details and customer care. We saved a lot of money by having professional advice from the start. Project management and quality at Craftex are at the highest level. Many thanks to Craftex Team.
How we design and build extensions
Many extension projects stall in the gap between an architect's drawings and a builder's price. Ours do not, because our design team and our construction team work in one company. Each option is priced as it is drawn, so the extension you approve is one you can afford.
In Hampstead, one client came to us with a house to renovate. At the first consultation we found room for a loft conversion and a rear extension. Our design team then worked with them through to planning approval, and our builders began the strip-out early to save time on the programme.
We reply to every enquiry the same working day. Ready to plan yours? Book a free design consultation and we will look at the house with you.
