A single-storey rear extension is the most commonly built home improvement in the UK — and for straightforward reasons. It typically does not need a planning application, it transforms the kitchen and living arrangements of the house, and the structural work is well-understood by contractors across the country. The question for most homeowners is not whether to build one but how large it can be and what the build will cost.
Permitted Development — the Size Limits Most Homeowners Start With
Under permitted development rights in England, a single-storey rear extension can extend up to 3 metres from the original rear wall on a semi-detached or terraced house, or 4 metres on a detached house, without a planning application. Maximum eaves height is 3 metres; maximum overall height is 4 metres. These are the baseline limits that apply automatically to most residential properties — no application, no fee, no waiting for a decision. However, properties in Conservation Areas, AONBs, and National Parks have restricted PD rights, and some newer-build estates have PD rights removed by planning condition.
"Permitted development is the entry point for rear extension planning, but it is the prior approval scheme — the larger home extension scheme — that changed what most homeowners can build without a full planning application. The extension of limits from 3m to 6m and from 4m to 8m under prior approval is one of the most significant practical planning policy changes for residential extensions in the last decade."— Planning Portal — Single-Storey Extensions and Permitted Development
The Prior Approval Scheme — Larger Extensions Without Full Planning
The larger home extension scheme (prior approval) allows single-storey rear extensions of up to 6 metres on a semi-detached or terraced house and 8 metres on a detached house — double the basic PD limits — provided the local planning authority is notified and raises no objection following neighbour consultation. This is not a full planning application: the process is faster, cheaper, and considers only the impact on neighbouring amenity (not design, not materials, not character). Most prior approval applications proceed without objection. The result is that most single-storey rear extensions in England that are planned to exceed the basic PD limits can still proceed without a full planning application, subject to the neighbour consultation outcome.
Building Regulations — Always Required
Planning permission and building regulations are entirely separate requirements. Every rear extension — including those built under permitted development — requires building regulations approval. The regulations cover: structural adequacy (foundations, walls, roof); thermal insulation (the extension must not fall below current U-value standards); drainage (if the extension affects existing drains); electrical installation; and damp-proofing. The completion certificate issued after final inspection is essential — it is required by solicitors on any subsequent property sale.
"The completion certificate for a single-storey extension is the document that confirms the build was inspected and met the required standards. It is routinely requested during conveyancing. An extension built without building regulations approval — or where the builder did not call for inspections — creates a problem on sale that is expensive and disruptive to resolve retrospectively."— GOV.UK — Building Regulations Approval
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