Most homeowners meet extension drawings backwards: a builder asks "have you got plans?", a council asks for documents by name, and suddenly you're pricing architects without knowing what you're buying. This page sets out exactly what each drawing set is for, what it should cost in 2026, and the order to buy them in — because paying for construction-grade drawings before planning is decided is one of the most common ways to waste money on an extension.
What drawings does a house extension need?
Three things, in this order:
- Planning drawings — scaled existing and proposed floor plans, elevations and roof plans (usually 1:50 or 1:100), plus a site/block plan. These show the council what the extension looks like and where it sits. They accompany a planning application or, for permitted development schemes, a lawful development certificate application.
- Building regulations drawings — the technical set: construction build-ups, insulation and thermal specification, drainage, ventilation, fire safety and structural openings. These show building control how it will be built, and they're what your builder actually prices and works from.
- Structural calculations — the engineer's justification for every steel, lintel and foundation. Almost every open-plan scheme needs them, and building control won't sign off without them.
Planning drawings win the permission; building regs drawings and calcs get it built and signed off. They are different documents at different levels of detail, and a set that tries to be both usually does neither well.
Planning drawings vs building regulations drawings
| Planning drawings | Building regs drawings | |
|---|---|---|
| Purpose | Win planning permission or certificate lawful development | Comply with the Building Regulations; what the builder prices and builds from |
| Audience | Council planning officer, neighbours | Building control inspector, structural engineer, builder |
| Detail level | Appearance, dimensions, materials named | Construction build-ups, U-values, drainage runs, structural detail |
| Typical 2026 cost | £1,200 – £2,500 | £1,500 – £3,000 |
| When to buy | First | Only after planning is secured (or the PD route is confirmed) |
The order matters. If planning is refused or amended, construction drawings done early get redrawn at your cost. Every figure above sits alongside the council's own charges — the £548 householder application fee and the rest are broken down in our planning fees guide.
What extension drawings cost in 2026
Realistic North London figures for a typical rear or side extension:
| Item | Typical cost | Notes |
|---|---|---|
| Planning drawing set | £1,200 – £2,500 | Existing + proposed plans, elevations, site plan, application handling |
| Building regulations set | £1,500 – £3,000 | Full construction specification and detail |
| Structural calculations | £500 – £1,500 | Per the engineer's scope; steels drive it |
| With The Extension Company's design & build | £0 on top | Every drawing above is inside the fixed price |
Bigger schemes — double storey, wrap-arounds, whole-house programmes — carry more drawing time, but the structure of the costs is the same.
Included free when we design and build
Our model is one fixed price for the whole project: survey, design, planning drawings, the application itself, building regs drawings, structural calculations and the build. There is no separate architect's bill, and the drawings are produced by the people who will build from them — which is why our details get approved and our sites don't stall on "drawing queries". We track what Enfield, Barnet and Haringey planning departments are approving street by street, and we draw to what gets consent.
Just need the drawings?
That works too. If you have a builder you trust, or you want permission in hand before choosing one, we produce planning and building regs packages as a standalone service across North London and the Hertfordshire border. You get the same council-literate drawings, the application handled, and a fixed quote for the set — with no obligation to build with us. Start with a free site visit or price the project first in the Extension Builder.
How the process works
- Measured survey — we survey the property and produce the existing drawings.
- Design — proposed drawings developed with you, against your budget (sanity-check it with the cost guide) and against what your council approves.
- Planning set submitted — full application or lawful development certificate, handled by us, council fee at cost.
- Building regs set + calcs — produced once the permission route is secure, then submitted to building control.
Allow two to three weeks from survey to a submitted planning set, and the council's statutory eight weeks for a decision.
Frequently asked questions
How much do planning drawings cost for an extension? Typically £1,200 to £2,500 in North London in 2026 for existing and proposed plans, elevations and a site plan with the application handled. Included free inside our fixed design-and-build price.
What are building regulations drawings? The technical construction set — build-ups, insulation values, drainage, fire safety and structural detail — that building control approves and your builder builds from. Planning drawings alone are not enough to build legally.
Do I need an architect for a house extension? You need competent scaled drawings, not necessarily an architect. Design-and-build firms, architectural designers and technicians all produce council-accepted sets. What matters is local planning literacy — drawings shaped to what your council actually approves.
Can I use your drawings with my own builder? Yes. Our standalone drawings service comes with no obligation to build with us, and the building regs set is detailed enough for any competent builder to price and build from.
How long do extension drawings take? Around two to three weeks from measured survey to a submitted planning set, then the building regs set is typically ready inside two weeks once permission is secured.
Do drawings include the structural calculations? They're a separate engineer's document, typically £500 to £1,500. We coordinate them as part of the building regs stage — and inside our design-and-build price they're included.