Building & projects

Building Regulations for Extensions

Name: TradePlanr
Rating: 5 (100 reviews)
Author: TradePlanr

Planning permission and building regulations are two different approvals, and an extension usually needs to satisfy both. Building control is about how it is built — and it is not optional.

Updated 23 Aug 20269 min read

It is a common and costly misunderstanding: planning permission says you may build an extension, but building regulations govern how it must be built to be safe, warm and sound. An extension almost always has to satisfy building control regardless of whether it needed planning permission.

This guide covers which Approved Documents apply to a typical domestic extension, the two routes through building control, what gets inspected, and the completion certificate that proves the job was signed off.

Key takeaways

Planning permission and building regulations are separate approvals — an extension usually needs to meet both.
Several Approved Documents apply: structure, fire, energy, ventilation, drainage, and more.
You can go through building control by full plans application or by building notice.
Building control inspects at key stages — foundations, drainage, structure — not just at the end.
The completion certificate is the proof of compliance the homeowner will need when they sell.

Planning vs building regulations

These two approvals answer different questions. Planning permission (or permitted development) decides whether the extension is acceptable in principle — its size, appearance and impact on neighbours. Building regulations decide whether it is built properly — structurally sound, weathertight, insulated, ventilated and safe.

An extension can be permitted development and still need full building control approval. Never let a customer assume that because they do not need planning, they do not need building regs — the building work itself almost always does.

Which Approved Documents apply

A typical single-storey rear extension touches most of the Approved Documents. Part A covers structure — foundations, beams and load paths. Part L covers conservation of fuel and power, driving the insulation and glazing. Part B covers fire safety, Part F ventilation, Part H drainage, Part K protection from falling, Part M access, and Part P any electrical work.

You do not memorise every clause, but you do need to know which parts your extension engages and design and build to them. Getting Part L insulation levels or Part H drainage falls wrong is a classic reason for a job to fail inspection.

Part L and Part H catch people out

Under-insulate against Part L or get the drainage falls and connections wrong against Part H and building control will not pass it. Design to the current Approved Documents from the outset, not as an afterthought.

Full plans vs building notice

There are two ways through building control. A full plans application submits detailed drawings and calculations for approval before work starts, so you get sign-off on the design and fewer surprises on site — the safer route for anything structural or complex. A building notice lets you start with minimal paperwork, relying on inspections as you go, which suits simple, well-understood work.

Full plans gives you and the customer certainty up front; a building notice is quicker to get going but puts more risk on site, because there is no approved design to fall back on if an inspector disagrees.

Inspections and sign-off

Building control inspects at key stages, not just at the end: commonly the foundation excavation before concrete, the drainage before it is covered, the damp-proof course, structural elements, insulation, and completion. You notify building control at each stage so they can inspect before the work is buried.

When the work passes final inspection, building control issues a completion certificate. That certificate is the homeowner's proof the extension was built to the regulations, and it will be asked for when they come to sell — so it is not paperwork to skip.

Keep the compliance trail on the job

TradePlanr keeps your inspection notes, photos and any electrical or other certificates stored against the job, so the compliance trail for the extension is organised in one place from foundations to completion.

Frequently asked questions

Do I need building regulations if the extension is permitted development?

Almost always yes. Permitted development or planning permission decides whether you may build; building regulations govern how it must be built. The construction work itself needs building control approval regardless of the planning route.

What is the difference between full plans and a building notice?

A full plans application gets the design approved before work starts, giving certainty and fewer surprises — best for structural or complex work. A building notice lets you start quickly with minimal paperwork and rely on site inspections, which suits simple, well-understood jobs.

Why does the completion certificate matter?

It is the proof the extension was built to the building regulations. The homeowner will be asked for it when they sell the property, so failing to obtain it can cause real problems down the line.

From guidance to action

TradePlanr for buildersKeep quotes, variations, certificates and job records in one place.Certificates on your phoneIssue and store the certificates a compliant job produces.Job diary & recordsStore inspection notes and photos against the job through every stage.