Regulations7 minute read
Do I need building regulations approval for an extension?
Almost always yes, and it is a separate approval from planning permission. Here is which route to take, and the certificate that matters in eight years' time.
The short answer
Yes, for practically every extension in England and Wales. Building regulations cover whether the building is safe and efficient, and they are separate from planning permission: you can need one, the other, or both. You apply either by full plans, which is approval before you start, or by a building notice, which is inspection as you go. Budget £800–£1,400 for a single-storey extension.
01Two different approvals, doing two different jobs
Planning permission is about what the building looks like and how it affects other people. Building regulations are about whether it stands up, keeps heat in and lets you out in a fire.
They are decided by different teams, on different timescales, against different rules. A rear extension that is permitted development still needs full building control sign-off.
02The short list of work that is exempt
- —A conservatory or porch under 30 square metres, at ground level, separated from the house by external-quality walls, doors or windows, with the heating on separate controls. Remove those doors and it stops being exempt.
- —A detached single-storey building under 30 square metres, containing no sleeping accommodation, and either more than one metre from a boundary or built substantially of non-combustible material.
- —A carport open on at least two sides, under 30 square metres.
- —Most repairs and like-for-like replacement, though replacing windows, a boiler or a flat roof covering is controlled work.
Everything else is in. An open-plan knock-through, a loft conversion, a rear extension, a new circuit or a garage conversion all need approval.
03Full plans or a building notice
| Full plans, decision period | 5 weeks, or 8 by agreement |
|---|---|
| Building notice, notice before starting | 2 working days |
| Completion certificate, after final inspection | 8 weeks |
| Regularisation, for work already built | no fixed period |
Full plans means drawings are checked and approved before anyone digs. It costs the same, takes longer at the start, and removes the argument on site.
A building notice suits small, familiar work with a builder you trust. On anything structural it moves the risk onto you, because nothing was approved in advance.
04The inspections, and what it costs
Your builder calls the inspector at set stages. Covering up work before it is seen is the one mistake that gets things dug back open.
- —Excavation, before concrete goes in.
- —Foundation concrete, and the damp-proof course.
- —Drains laid, before backfilling, and again for the water test.
- —Structural steels and their bearings, before they are boxed in.
- —Insulation and the airtightness detail, before plastering.
- —Completion, which triggers the certificate.
| Single-storey extension, up to 40 sqm | £800–£1,400 |
|---|---|
| Loft conversion | £800–£1,400 |
| Two-storey extension | £1,200–£2,000 |
| Regularisation of existing work | £1,000–£2,500 |
05What it drags in from the rest of the house
This is the part that surprises people. An extension is assessed as part of the whole dwelling, so the work reaches back into the existing building.
- —Escape windows in the rooms you affect, and smoke alarms wired through the house.
- —Thermal upgrades to any element you disturb, so a wall you open up has to meet the current standard.
- —Electrical work certified under Part P, by a registered electrician, on a certificate you keep.
- —A structural opening in the existing wall, designed and inspected, even though the room itself is not new.
None of this is a surprise to a building inspector. It is a surprise to a homeowner reading a quote that only priced the new part.
Common questions
- What happens if I build an extension without building regulations approval?
- The council can require alterations, and in some cases removal, for up to a year after completion. The lasting problem is the sale: with no certificate, a buyer's solicitor treats the work as a defect.
- Can I get building regulations approval after the work is finished?
- Yes, through a regularisation application to the local authority. You pay a higher fee and expect to open up work so it can be inspected. A registered building control approver cannot regularise past work.
- Do I need building regulations approval for a conservatory?
- Usually not, if it is under 30 square metres at ground level, separated from the house by external-quality doors or windows, and has heating on independent controls. Remove the separating doors and approval is needed.
Rules described here apply in England and Wales, and were last checked on 29 August 2026. Permitted development limits and fees change. Check your council’s current guidance before you rely on a figure.