How Far Can You Extend Your House Without Planning Permission?

The maximum depth of a single-storey rear extension under standard permitted development:

Depth is measured from the rear wall of the original house (the house as first built, or as it stood on 1 July 1948). Any existing extensions count towards these limits.

The prior approval route costs £249 and allows significantly deeper extensions. Your council notifies neighbours, who have 21 days to comment. The decision is based solely on impact on adjoining amenity.

A single-storey side extension must not exceed half the width of the original house. It must also:

• Not exceed 4m in height
• Not be on the side of a property on designated land
• Not front a highway (different rules apply for front extensions, which are generally not PD)

Two-storey side extensions are more restricted: they must not be within 2m of a side boundary and must use materials that match the existing house.

Two-storey rear extensions are permitted development with these limits:

• Maximum depth: 3m (all house types)
• Minimum distance from rear boundary: 7m
• Minimum distance from side boundary: 2m (for two-storey side extensions)
• Maximum eaves height: must not exceed the eaves of the original house
• Materials: must be similar in appearance to the existing house

Two-storey extensions are not permitted development on terraced houses or on any property on designated land.

Height is one of the most commonly misunderstood PD rules. The limits are:

• Single-storey extension max height: 4m overall
• Single-storey extension max eaves height: 3m (must not exceed eaves of original house)
• Two-storey extension: eaves and ridge must not exceed those of the original house
• Within 2m of a boundary: maximum overall height of 3m (this applies to outbuildings too)

Height is measured from ground level at the base of the extension, not from the floor level of the original house. On sloping sites this can be significant.

The total area of ground covered by extensions, outbuildings, and other structures (excluding the original house) must not exceed 50% of the total curtilage area.

This means you need to add up the footprint of all extensions, sheds, garages, outbuildings, and any other structures on the property. If the total exceeds 50% of the curtilage, the development is not permitted development.

This is a cumulative limit that includes work by previous owners. If a previous extension already covers 40% of the garden, you only have 10% remaining.

Remember that even if your extension is within PD limits, you will still need building regulations approval and may need to serve party wall notices if building near a neighbour’s property.

All PD measurements are taken from the original house, defined as the house as it was first built, or as it stood on 1 July 1948 if built before that date.

This means:

• Extensions added by previous owners reduce your remaining allowances
• A conservatory built in the 1990s counts as an existing extension, even if you plan to demolish it
• If you demolish an existing extension first, the replacement is still measured from the original rear wall

Finding the original house dimensions can be tricky. Old Ordnance Survey maps, the original planning permission, or your local council’s records can help establish the original footprint.

If your project exceeds permitted development limits, you have several options:

• Prior approval — for single-storey rear extensions up to 8m (detached) or 6m (semi/terrace). Costs £249. See our prior approval guide
• Full planning permission — apply through the Planning Portal. Costs £548 for a householder application and takes around 8 weeks. See our cost guide
• Pre-application advice — pay your council for informal advice before submitting a full application. Typically £100–£300