Permitted Development Rights for Extensions: How Big Can You Build?

Extensions are one of the most common projects homeowners try to plan under permitted development, and one of the most common places people get the detail wrong — the size you can build without full planning permission depends on several factors at once: house type, how far you extend, how high you build, and whether you use the standard allowance or the larger "prior approval" route.

The general size picture

As a broad guide (always confirm current, exact figures for your property):

• Single-storey rear extension, detached house: up to 4m depth under the standard allowance.
• Single-storey rear extension, semi-detached or terraced house: up to 3m depth under the standard allowance — a smaller allowance than a detached house gets for an identical-looking extension.
• Larger Home Extension (prior approval) route: extends the single-storey depth allowance to 8m (detached) or 6m (semi-detached/terraced), for a defined period, subject to your local authority notifying neighbours and confirming no valid objection.
• Maximum height: single-storey rear extensions under permitted development generally have a 4m height limit, reduced to a lower eaves height (around 3m) where the extension is within 2m of a boundary.
• Two-storey rear extensions: a smaller, more restrictive allowance — broadly around 3m of depth, and not permitted within 7m of the rear boundary (a rule specifically designed to protect neighbours' light and privacy).

Worked examples — the same rule, different outcomes

• A detached house building a single-storey rear extension can generally go up to 4m under the standard allowance, or 8m via prior approval — noticeably more room to work with than an otherwise-identical semi-detached or terraced house next door.
• A semi-detached or terraced house doing the same project is generally capped at 3m under the standard allowance, or 6m via prior approval — the smaller figure reflects the closer proximity to neighbouring properties on both projects.
• The same semi-detached house attempting a two-storey rear extension instead faces a tighter limit again — roughly 3m depth, and it must stay outside 7m of the rear boundary, regardless of the single-storey allowance it could otherwise use.

These figures move and get reviewed periodically, and the exact numbers matter a lot for whether your project genuinely qualifies — this is exactly the kind of thing to confirm directly with your local planning authority, or via a formal Certificate of Lawful Development, before committing to a design. See our general guide on permitted development for the wider system this sits within.

How the prior approval (Larger Home Extension) process works

• You submit a prior approval application to your local authority, describing the larger extension
• The council writes to your immediate neighbours, giving them a defined period to raise an objection
• If a neighbour objects, the council assesses the impact on their amenity (light, privacy, outlook) before deciding
• If no valid objection is raised, or the council decides the impact is acceptable, the larger extension is confirmed as permitted development — without it being treated as a full planning application

What can rule permitted development out entirely

• An Article 4 direction removing extension permitted development rights in your area
• A listed building, or a condition on a previous planning permission
• Being in a conservation area, National Park or Area of Outstanding Natural Beauty, where allowances are often reduced
• The extension covering more than a set proportion of the garden/curtilage relative to the original house
• A previous extension already having used up part of your property's permitted development allowance

Don't forget the neighbour side

Extending close to a shared boundary or an existing party wall commonly triggers a separate legal requirement to formally notify your neighbour, independent of planning permission — see our guide on Party Wall Agreements.

Building regulations still apply

Whether or not planning permission is needed, an extension almost always needs building regulations approval — structure, insulation, drainage, fire safety and more all get checked separately from the planning question. See our guide on building regulations for extensions for what that covers, and our single-storey extension cost guide for what a project like this typically costs once you know what you can build.

Before you finalise a design: a short checklist

• Confirm your exact house type — detached vs semi-detached/terraced makes a real difference to your allowance
• Check whether a previous extension has already used part of your allowance
• Decide whether the standard allowance is enough, or whether you'd need the prior approval route
• Check your designated-area status (conservation area, Article 4, listed building)
• Confirm whether a party wall notice will be needed for the specific boundary you're building near
• Get the current, exact figures confirmed with your local planning authority before finalising a design

Getting it right before you design

Because the size limits are genuinely specific and property-dependent, the sensible order of operations is: confirm what you can build under permitted development (or whether you need full planning permission) before finalising a design, not after. A Certificate of Lawful Development is worth the modest cost for the certainty it gives you, especially if you plan to sell the property in future.