UK Extension Rules – Simple Guide for Homeowners

Thinking about adding a sunroom, loft, or a bigger kitchen? The first thing most people hit is the maze of rules that decide whether you can start building tomorrow or need a council meeting. This guide breaks down the essential UK extension rules you need to know, so you can avoid costly delays and get your project moving.

When Do You Need Planning Permission?

In England, most small extensions fall under Permitted Development (PD) – that means you can build without a full planning application as long as you stay within the limits. The main limits are:

• Extension must not exceed 4 metres for a detached house or 3 metres for attached, semi‑detached and terraced homes.
• The new part can’t be higher than the existing roof line.
• Side extensions should stay within 3 metres of the original side wall (or 4 metres if you have a remote rear garden).
• Total floor area added can’t be more than 50% of the original house floor area (or 40% for listed buildings).

If your plans break any of those numbers, you’ll need to submit a full planning application. Scotland, Wales and Northern Ireland have similar but slightly different thresholds, so double‑check the local authority guidelines.

Common Mistakes That Delay Approval

Even when you stay inside the PD limits, a few slip‑ups can stall your project.

• Party wall issues: If the extension touches a neighbour’s wall, you must serve a Party Wall Notice and wait for their response.
• Neighbour objections: Ignoring a neighbour’s concerns about loss of light or view can lead to a formal complaint and delay.
• Fire safety: Adding a new storey or loft may require fire‑resistant materials and proper escape routes.
• Structural calculations: Skipping a structural engineer’s sign‑off can result in the council rejecting your plans.
• Incorrect documents: Forgetting the required drawings, site plans or heritage statements (for listed buildings) sends your application straight back.

To keep things smooth, gather all paperwork early and talk to a qualified architect or builder who knows the local council’s expectations.

Once you have the paperwork sorted, submit your application online through the Planning Portal. You’ll usually hear back within 8 weeks for a straightforward case. If the council asks for more info, respond quickly – every extra day adds to your timeline.

Finally, remember that even after approval you must follow building regulations for structural safety, insulation, and energy efficiency. Those rules are separate from planning permission but are just as important for a legal, safe extension.

By staying on top of the UK extension rules, you’ll save time, money, and a lot of headaches. Ready to start drawing? Grab a sketchpad, check the PD limits, and give your local council a quick call to confirm any specifics for your area.