Extending your home can add space, value, and long-term flexibility, but only if it’s done legally. In the UK, home extensions are governed by a mix of planning law, building regulations, and property-specific legal constraints. Confusing these rules is one of the most common (and expensive) mistakes homeowners make.
This guide explains exactly what you need to know before you build.
Planning permission controls whether an extension is allowed, based on its size, location, and impact on neighbours and the wider area.
Not every extension needs planning permission, but the rules are strict.
Many home extensions fall under Permitted Development (PD) rights, meaning formal planning permission isn’t required, provided all criteria are met.
If any rule is breached, planning permission is required.
Your extension may qualify as permitted development if it:
Previous extensions, even by former owners, count towards your allowance.
|
Property Type |
Standard PD Limit |
|
Terraced |
Up to 3m |
|
Semi-detached |
Up to 3m |
|
Detached |
Up to 4m |
Measured from the rear wall of the original house, not later additions.
You may be able to extend further using the Prior Approval process:
|
Property Type |
Maximum Projection |
|
Terraced / Semi |
Up to 6m |
|
Detached |
Up to 8m |
Prior Approval is a formal process and should not be treated as a formality.
Prior Approval includes neighbour consultation and formal assessment by the local authority, so the proposal must meet all technical criteria to avoid refusal
PD rights are restricted or removed if the property is:
Building regulations control how an extension is built, not whether it is allowed.
Even if your extension does not require planning permission, building regulations approval is almost always required.
These rules exist to ensure extensions are safe, structurally sound, energy efficient, and suitable for long-term use.
Building regulations approval is required for most home extensions, including:
Minor works such as certain small porches or conservatories may be exempt if they meet strict size, separation and heating criteria. These exemptions are specific and should always be confirmed with building control before work begins.
Building control inspectors assess compliance at key stages of the build.
|
Area |
What’s Assessed |
|
Structure |
Foundations, steel beams (RSJs), load paths |
|
Fire Safety |
Escape routes, alarms, fire separation |
|
Thermal Performance |
Insulation, airtightness, ventilation |
|
Drainage |
Foul and surface water systems |
These checks ensure the extension meets national safety and performance standards.
Building control inspections take place at key stages of the build, allowing inspectors to check that work is being carried out in line with approved plans and national building regulations.
Typical inspection stages include:
These inspections are not designed to delay the project, but to identify issues early, when they are easier and cheaper to resolve.
If problems are identified, building control may require corrective or remedial work before construction continues or before a completion certificate is issued. This can include adjustments to structure, insulation, fire safety measures, or drainage details.
Once all stages have passed and any required changes are addressed, building control will issue a completion certificate, confirming the extension meets current building regulations.
Once the extension fully complies with building regulations, a completion certificate is issued by building control. This document confirms that the work has been inspected and meets national safety and performance standards.
The completion certificate is often requested when:
Without it, buyers and lenders may raise concerns about the legality and safety of the extension. This can lead to delays, requests for retrospective approval, price renegotiations, or, in some cases, aborted transactions.
Keeping this documentation safe ensures the extension is recognised as compliant and helps protect the value and marketability of your home.
In addition to planning permission and building regulations, home extensions are subject to legal obligations that sit outside the planning system. These relate to neighbour rights, property ownership, and third-party consents.
Failing to address these can lead to disputes, delays, or legal action, even where planning and building control approvals are in place.
The Party Wall Act exists to protect neighbouring properties during building works.
You must serve formal notice if your extension involves:
Neighbours may:
Planning permission does not remove party wall obligations.
Some properties are subject to restrictive covenants recorded in the title deeds.
These can restrict or control:
|
Important Point |
What It Means |
|
Planning permission granted |
Does not cancel covenants |
|
Covenant breached |
Can still be enforced privately |
|
Enforcement risk |
May arise years later, including on sale |
Covenants are private legal matters and should be checked before work begins.
If the property is leasehold, extending almost always requires:
Failure to obtain consent can:
This applies even where planning and building control approvals exist.
Additional legal permissions may be required if the extension affects:
These approvals are issued by utility providers or statutory bodies and sit outside the planning system.
If legal requirements were missed:
|
Common Assumption |
Reality |
|
“Planning approval covers everything” |
❌ It doesn’t |
|
“Indemnity insurance fixes it” |
❌ It only limits risk |
|
“No one will notice” |
❌ Issues often arise on sale |
Indemnity insurance does not correct non-compliant work and has strict limitations.
If you’re unsure which rules apply to your extension, whether planning permission, building regulations, or legal requirements, getting advice early can make a significant difference. A short conversation at the outset can help clarify what approvals are needed, avoid costly mistakes, and keep your project moving smoothly from design to completion.