One of the most common questions UK homeowners ask is: how big can I extend my house without planning permission? The good news is that permitted development rights allow you to add significant space to your home — but the exact limits depend on your house type, the type of extension, and where you live.

This guide sets out the current size limits for the most common extension types under permitted development (PD) rules in England.

What Is the Maximum Size for a Rear Extension Under Permitted Development?

Rear extensions are the most popular type of home extension in the UK, and permitted development rules allow for generous sizes — particularly under the Neighbour Consultation Scheme (also called the Prior Approval process).

Single Storey Rear Extensions

House Type	Standard PD Limit	With Prior Approval (Neighbour Consultation Scheme)
Detached house	4 metres	Up to 8 metres
Semi-detached or terraced	3 metres	Up to 6 metres

The extension must not exceed 4 metres in height, and must not be higher than the eaves of the existing roof.

Double Storey Rear Extensions

Double storey rear extensions have tighter limits under PD:

• Maximum depth of 3 metres from the original rear wall
• Must be at least 7 metres from the rear boundary
• Maximum height must not exceed the height of the existing roof ridge
• Roof pitch must match the existing house as far as practicable

What Is the Maximum Size for a Side Extension?

Single storey side extensions can be built under permitted development provided they meet all of the following conditions:

• Maximum width of half the width of the original house
• Maximum height of 4 metres
• Must be single storey only (double storey side extensions require planning permission)
• Must not front a highway (road or public footpath)

So if your original house is 8 metres wide, your side extension cannot be wider than 4 metres.

What Are the Size Limits for a Loft Conversion?

Loft conversions are measured in terms of the additional volume they add to the roof space:

House Type	Maximum Additional Volume
Terraced house	40 cubic metres
Detached or semi-detached	50 cubic metres

The roof extension must not exceed the highest part of the existing roof, and any side-facing windows must be obscure-glazed.

What Are the Size Limits for a Porch?

• Maximum ground floor area: 3 square metres
• Maximum height: 3 metres
• Must be more than 2 metres from the boundary with a highway

What Are the Size Limits for Outbuildings and Garden Rooms?

Outbuildings, sheds, home offices, and garden rooms fall under a separate permitted development class. The key rules are:

• Must not cover more than 50% of the garden area (when combined with any existing outbuildings)
• Maximum eaves height: 2.5 metres
• Maximum overall height: 4 metres (dual pitched roof) or 3 metres (any other roof)
• If within 2 metres of a boundary, maximum height is 2.5 metres
• Must not be forward of the principal elevation (front of the house)
• Must not be used as a separate dwelling (e.g. rented out)

Key Rules That Apply to All Extensions

Regardless of the type of extension, all permitted development projects must comply with these overarching rules:

• 50% garden coverage rule — Extensions and outbuildings combined must not cover more than half the land around the original house
• Original house measurements — All limits are measured from the original house as built (or as it stood on 1 July 1948), not the current footprint after any previous extensions
• Materials must match — The extension should be built in materials that closely match the appearance of the existing house
• No elevated platforms — Verandas, balconies, and raised platforms are not permitted development

When Do These Limits NOT Apply?

These permitted development size limits apply to standard residential houses in England. They do not apply if:

• Your home is in a conservation area or National Park (reduced PD rights apply)
• Your home is a listed building (separate listed building consent required)
• An Article 4 Direction has removed PD rights in your area
• Your home is a flat or maisonette
• Your property is subject to a planning condition removing PD rights

Do You Still Need Drawings and Building Regulations Approval?

Yes. Even when your extension qualifies under permitted development, you will still need building regulations approval before construction begins. This requires a full set of architectural drawings showing the layout, dimensions, structural details, and materials of the proposed work.

At Permitted Development, we provide ready-to-use architectural drawing packages for the most common extension types — all compliant with UK building regulations and verified against current PD rules. Available as instant downloads.

Frequently Asked Questions

Can I extend my house by 6 metres without planning permission?

Yes — if you have a semi-detached or terraced house and apply through the Neighbour Consultation Scheme (Prior Approval), you can extend up to 6 metres to the rear without full planning permission. Detached houses can go up to 8 metres.

Does a previous extension affect my permitted development allowance?

Yes. All size limits are based on the original house, not the current one. If a previous owner already extended the property, that extension counts towards your PD allowance.

How do I measure the depth of a rear extension?

The depth is measured from the original rear wall of the house — not from any existing extensions. So if a conservatory was already added, the measurement starts from the original wall, not the back of the conservatory.

Is there a maximum height for a single storey extension?

Yes. A single storey rear extension must not exceed 4 metres in height and must not be taller than the eaves of the existing roof.