Do I Need Planning Permission for a Rear Extension?

Every permitted development rule is measured against the original house — the house as it was first built, or as it stood on 1 July 1948, whichever is later. This is a critical concept because it affects how depth, height, and coverage are calculated.

If a previous owner added an extension after that date, it still counts as an addition. You measure your permitted development allowance from the original rear wall, not from any later extension. That previous extension also counts towards your 50% coverage limit. This catches many homeowners out: just because an extension is already there does not mean you have the same allowance left.

If you are unsure where the original rear wall is, your local authority holds records of past planning applications and building control approvals. Your title deeds or the original OS map for your property can also help.

Under standard permitted development, the maximum depth of a single-storey rear extension depends on your house type:

• Detached houses — 4 metres from the original rear wall
• Semi-detached and terraced houses — 3 metres from the original rear wall

These are the limits you can build to without any notification, consultation, or approval from the council. You simply need to ensure every other Class A rule is also met.

Depth is measured along the ground from the rear wall of the original house to the back wall of the extension. If the rear wall is staggered (some houses have an L-shaped plan), the measurement is taken from the part of the rear wall that the extension projects beyond.

If you need to go deeper than the standard limits, the Larger Home Extension scheme may help. Introduced in 2013 and made permanent in 2019, it increases the permitted depth to:

• Detached houses — up to 8 metres
• Semi-detached and terraced houses — up to 6 metres

This is not an automatic right. You must apply to your local planning authority for prior approval before starting work. The process works like this:

1. You submit a prior approval application (the fee is currently £120).
2. The council notifies your immediate neighbours, who have 21 days to comment.
3. The council then has 42 days from the date of your application to make a decision.
4. If no objection is raised and the council does not refuse, you can proceed.
5. If the council does not respond at all within 42 days, approval is deemed granted.

The prior approval assessment only considers the impact on the amenity of adjoining properties — mainly overlooking, loss of light, and visual impact. The council cannot refuse on design, materials, or other planning grounds. This makes it a much lighter process than full planning permission.

Two-storey rear extensions can also be permitted development under Class A, but the rules are tighter:

• The maximum depth is 3 metres from the original rear wall, regardless of house type.
• The extension must not come within 7 metres of the rear boundary.
• The ridge height must not exceed the ridge height of the original house.
• The eaves must match the eaves height of the existing house.
• The roof pitch must match the existing roof pitch as far as practicable.
• Any upper-floor window in a side elevation must be obscure glazed and non-opening (or top-hung opening above 1.7m).

Double-storey extensions are not permitted development on designated land (conservation areas, national parks, AONBs, etc.). On designated land, only single-storey extensions are allowed under Class A.

For single-storey rear extensions, the height rules are:

• Maximum overall height — 4 metres
• Eaves height within 2 metres of a boundary — maximum 3 metres

The 4-metre height limit is measured from the natural ground level at the base of the extension to the highest point of the roof (including any parapet). The eaves restriction within 2 metres of a boundary is one of the most commonly encountered rules, particularly on narrow plots where the extension runs close to a side boundary.

If your garden slopes, the measurement is taken from the natural ground level directly beneath the part of the extension being measured, not from the highest or lowest point of the slope. On steeply sloping sites, this can create complications where different parts of the extension have different effective heights.

No more than half the area of land around the original house can be covered by additions or other buildings. This includes:

• All extensions (rear, side, double-storey)
• All outbuildings (sheds, garages, garden offices)
• Any other structures that count as development

The calculation excludes the footprint of the original house itself. It covers the entire curtilage of the property, including the front, side, and rear garden. Many homeowners underestimate this limit, particularly if there are already multiple outbuildings on site.

A permitted development rear extension must also satisfy these conditions:

• Materials must be similar in appearance to the existing house. You do not need to match the exact brick, but the colour, texture and general character should be consistent. Render-over-block is generally acceptable if the existing house is rendered.
• No veranda, balcony, or raised platform can form part of the extension. A raised patio more than 300mm above ground level is treated as a raised platform and falls outside PD.
• The extension must not extend beyond the side elevation of the original house (a side extension is covered by Class A too, but with its own rules).

Single-storey side extensions are also covered by Class A. The additional rules for side extensions are:

• Maximum width of half the width of the original house.
• Maximum height of 4 metres.
• If within 2 metres of a boundary, maximum eaves height of 3 metres.

On designated land (conservation areas, national parks, AONBs), side extensions are not permitted development at all. You will need full planning permission.

An Article 4 direction is a power that allows a local planning authority to remove specific permitted development rights in a defined area. If your property is subject to an Article 4 direction that covers Class A, you will need planning permission for any rear extension, regardless of size.

Article 4 directions are most commonly found in conservation areas, but they can apply anywhere. They are specific to each council area and can cover different classes of permitted development. You can check whether your property is affected by searching your council’s planning policy pages or by calling their planning department.

If your property is in a conservation area, national park, Area of Outstanding Natural Beauty (AONB), World Heritage Site, or the Broads, additional restrictions apply under Class A:

• Side extensions are not permitted development.
• Double-storey rear extensions are not permitted development.
• Single-storey rear extensions can still be built under standard PD limits (3m or 4m depending on house type), but cladding materials on the exterior must be similar in appearance to the existing house.
• The Larger Home Extension (prior approval) route is still available on designated land.

Listed buildings do not benefit from any permitted development rights under Part 1. Any extension to a listed building requires both listed building consent and planning permission, regardless of size.

Planning permission and party wall notices are completely separate legal requirements. Even if your extension is permitted development, you may still need to serve a party wall notice under the Party Wall etc. Act 1996 if:

• You are building on or at the boundary line (Section 1).
• You are carrying out work to a shared wall, such as cutting in a steel beam or underpinning (Section 2).
• You are excavating within 3 metres of a neighbouring structure to a depth below their foundations, or within 6 metres where the excavation intersects a 45-degree line from their foundation base (Section 6).

Most rear extensions that involve new foundations near a neighbour will trigger at least Section 6. A steel beam bearing into a party wall triggers Section 2. Failing to serve the correct notices is a civil matter — your neighbour can seek an injunction to stop your work. Use our free Party Wall Calculator to check what applies.

Building regulations approval is almost always required for a rear extension, even if it qualifies as permitted development. Planning permission and building regulations are completely separate systems.

Building regulations cover:

• Structural safety — foundations, lintels, roof structure
• Thermal performance — insulation values for walls, roof, floor, and glazing
• Drainage — foul and surface water connections
• Fire safety — escape routes, fire-rated materials where needed
• Ventilation — trickle vents, extract fans, air permeability
• Electrical safety — new circuits, consumer unit upgrades

You can either apply directly to your local council’s building control team or use a private Approved Inspector. The building control process runs alongside the physical construction and involves inspections at key stages.

Permitted development is self-certifying — you do not need to apply for anything before you build. However, a Lawful Development Certificate (LDC) from your local planning authority provides formal written confirmation that your extension is lawful. It is the only official document that proves your extension did not need planning permission.

Most solicitors will ask for an LDC when you come to sell or remortgage. If you cannot produce one, the buyer’s solicitor may require indemnity insurance or ask you to apply retrospectively, both of which can delay a sale. Getting an LDC before or during construction is straightforward and avoids this problem entirely.

The current fee for an LDC for a proposed development is £258. For an existing development (already built), the fee is £258. The council has 8 weeks to determine the application.

If you are thinking about a rear extension, here is a practical sequence of steps:

1. Check your permitted development rights — use our free calculator to see if your extension falls within the depth limits. Check whether your property is in a conservation area or subject to an Article 4 direction.
2. Check party wall obligations — use our Party Wall Calculator to see if you need to serve notices on neighbours.
3. Get drawings — even for permitted development, you will need drawings for building regulations and for any builder to price the work accurately.
4. Apply for an LDC — optional but strongly recommended, especially if you plan to sell in the next 10 years.
5. Submit building regulations — either a Full Plans application or a Building Notice.
6. Serve party wall notices — if applicable, allow at least 1–2 months for the notice period before starting work.
7. Start building — once building regulations are approved (or a notice accepted) and any party wall matters are resolved.