The short answer
No. Nothing in UK law says an architect must design a house extension. You do need drawings good enough for building control, and often for the planning department, but a technologist, a building designer or the firm building the extension can produce them. An architect is worth paying for when the house is listed, the site is awkward or the design matters more than the budget.
On this page
What the law asks for
The law cares about the drawings, not about who made them. Two sets of rules can apply to an extension.
Planning comes first. Many single-storey rear extensions in England are permitted development and need no application at all. If yours does need one, the council wants scaled plans and elevations of the house as it is and as it will be. Anyone may draw them.
Building regulations always apply. Building control needs to see how the extension stands up, keeps heat in, resists fire and drains. For most extensions that means a full set of construction drawings and a structural engineer’s calculations for the foundations, the roof and any beam over the opening into the house.
So the real question is who is best placed to produce those two packages for your project.
Who can draw an extension
An architect. The title is protected by law. Only someone on the Architects Registration Board register may use it, which typically follows seven years of training. You can check a name on the register in a minute.
An architectural technologist. Chartered technologists specialise in how buildings go together. For a plain extension they do the same job as an architect, often for less.
A building designer or surveyor. Many local practices draw extensions all week. Quality varies, so ask to see approved drawings from recent jobs and check they carry professional indemnity insurance.
A design-and-build firm. Plenty of builders employ or hire a designer and fold the drawings into the build price. With a prefab extension, the maker’s own team usually produces the factory drawings, and some handle the planning and building control drawings as well.
What each route costs
Planning drawings for a single-storey extension typically cost £1,000 to £2,500. An architect on a full service usually charges 3% to 15% of the build cost; RIBA notes that architects may charge a percentage, a fixed fee or by the hour. A structural engineer costs £600 to £1,200 for a single storey and £800 to £1,800 for two storeys. Council building control for a single storey of 10 to 40 m² is about £700 to £1,100, and each council sets its own fees. These are typical national ranges for 2026, including VAT; London and the South East usually cost 20 to 40% more. A guide, not a quote.
| Who | What you get | Typical cost |
|---|---|---|
| Architect, full service | Design, planning, construction drawings, help choosing a builder | 3% to 15% of the build cost |
| Technologist or designer | Planning drawings | £1,000 to £2,500 |
| Design-and-build firm | Drawings included in the build contract | Folded into the price |
| Structural engineer | Calculations and structural details | £600 to £1,200 single storey, £800 to £1,800 two storeys |
You need the engineer whichever designer you choose. Very few designers do their own structural calculations.
When an architect earns the fee
Hire one, and a good one, in these cases.
The house is a listed building, or it sits inside a conservation area. The council will judge the design closely, and an architect who knows the planning officers saves months.
The plot is difficult. Steep ground, a boundary on the skew, a neighbour’s window a metre away or a mature tree all call for someone who can design around a problem.
The inside of the house is changing too. If the extension means moving the kitchen, the stairs or the bathroom, the value is in the layout of the whole floor, and that is what architects train for.
You want something out of the ordinary. Frameless glass corners, a roof that floats, a room that looks nothing like the house: these need careful detailing or they leak.
When you can manage without
A rectangular room on the back of an ordinary house is well-trodden ground. A technologist or an experienced designer will produce drawings that pass, and the money saved will pay for better doors.
Factory-made extensions narrow the design choices further. The maker works with set panel sizes, standard junctions and tested details, so much of the technical design already exists. You choose a size, a roof, the openings and the finishes. That is a large part of why prefab runs to a shorter programme.
One caution. If the firm that builds the extension also draws it, nobody independent is checking the builder. That works well with a reputable firm and a clear specification. If you would like a second pair of eyes, pay a designer by the hour to review the drawings before you sign.
Questions to ask any designer
- How many extensions like mine have you drawn in the last year?
- Is the structural engineer’s fee inside your quote or on top?
- Do you submit the planning and building control applications, and are the council fees included?
- What happens if planning is refused: do you charge for a revised scheme?
- Will you produce a written specification a builder can price from?
The fifth matters most. Vague drawings produce vague quotes, and the gaps come back later as extras.
What happens if you skip drawings altogether
For small works, building control will accept a building notice, which is a short form with no detailed plans. It is quicker to start. It also means the inspector judges the work as it is built, and anything that fails has to be taken apart. For an extension with foundations, a roof and a structural opening, full plans approved in advance are the safer route.
If you would like a price from a firm that designs and builds prefab extensions, the form below takes about a minute.