What Size Extension Can I Build Without Planning

Extending your home can be a cost-effective and practical way to increase space, improve functionality and add value. The good news for many homeowners in England is that certain extensions can be built without applying for planning permission, thanks to permitted development rights. However, there are strict limits on the size, height and location of what is allowed. Understanding these rules before starting your project can help you avoid delays, costly mistakes or enforcement action from the local authority.

Permitted Development for Extensions

Permitted development rights are a national set of rules that allow you to carry out certain types of building work without needing full planning permission. These rights apply to houses only, not flats, maisonettes or converted buildings. They can also be restricted by conditions on the property, such as those imposed by an Article 4 Direction or a planning condition attached to a previous consent. If your home is in a conservation area, national park or Area of Outstanding Natural Beauty, additional restrictions usually apply.

Single-Storey Rear Extensions

For most houses in England, a single-storey rear extension can be built without planning permission if it meets the following criteria. The extension must not extend more than three metres beyond the rear wall of the original house for a terraced or semi-detached property, or more than four metres for a detached house. The maximum height allowed is four metres, and eaves must not exceed three metres where they are within two metres of a boundary. These measurements apply to the original house as it stood on 1 July 1948, or when it was first built if later.

Since 2013, the government has allowed larger single-storey rear extensions of up to six metres for terraced and semi-detached houses, and up to eight metres for detached houses, under a temporary scheme. This was made permanent in 2020. However, this type of extension still requires prior approval through the Neighbour Consultation Scheme. This is not the same as full planning permission, but it involves notifying the local authority, who will consult your neighbours. If no objections are raised or the council deems the proposal acceptable, work can go ahead.

Two-Storey Extensions

Two-storey rear extensions are permitted under certain conditions. They must not extend more than three metres beyond the original rear wall, must not be within seven metres of the rear boundary, and must not exceed the height of the original house. The roof pitch of the extension must match the existing roof as closely as possible. Side-facing windows must be obscure glazed and non-opening below 1.7 metres from floor level. The materials must be similar in appearance to the existing building.

Side Extensions

Single-storey side extensions are also allowed under permitted development, provided they do not extend more than half the width of the original house. They must be single storey, no higher than four metres, and must not be within two metres of a boundary if the eaves exceed three metres. Side extensions cannot be built under permitted development if the property is in a conservation area or similar designation.

Wraparound and L-Shaped Extensions

A wraparound extension that combines a side and rear element is not normally considered permitted development, because it is treated as a single structure. Even if each part on its own would qualify, the combined footprint usually exceeds permitted development limits. These types of extensions typically require full planning permission, particularly when they change the character of the building or impact neighbouring properties.

Loft Extensions and Outbuildings

While not part of the main extension rules, it is worth noting that loft conversions and garden outbuildings also fall under permitted development. A dormer added to the rear of a house, for example, is allowed without permission if it does not exceed forty cubic metres for terraced houses or fifty cubic metres for detached and semi-detached homes. Garden buildings must not take up more than fifty per cent of the garden space and must meet height and placement limits.

Designated Areas and Restrictions

If your property is in a conservation area, national park, World Heritage Site or Area of Outstanding Natural Beauty, permitted development rights are often limited or removed. You may be unable to build any extensions without planning permission in these areas, and stricter design rules will usually apply. It is important to check with your local planning authority before proceeding.

Confirming Your Rights and Next Steps

Even if your proposed extension meets permitted development rules, it is strongly recommended to apply for a Certificate of Lawful Development. This is issued by the local authority to confirm that your work is legally permitted. It is especially useful when selling the property in future, as it proves the extension was compliant with planning rules at the time of construction.

Conclusion

In most cases, you can build a rear or side extension without planning permission if it falls within the permitted development limits on size, height and placement. Single-storey rear extensions up to three or four metres are typically allowed outright, and larger extensions up to eight metres may be allowed with prior approval. Two-storey and side extensions are more restricted and must meet specific criteria. Always check for local restrictions or previous planning conditions, and seek professional advice if you are unsure. A careful and informed approach will help ensure your extension adds space, value and comfort without planning complications.