When does a side extension need planning permission in England, and when is it permitted development? Understand the national width and height limits, why side extensions are more restricted, and how to check your property first.
Side extensions are treated more cautiously than rear extensions under England's national permitted development rights, because they change how a house meets the street and its neighbours. The width and height limits are tighter, and several common situations remove the right altogether.
These are the common features that keep a project on the simpler route.
These are the usual triggers that push a scheme beyond straightforward PD rights.
The tool is designed to answer the first question most homeowners have: is this worth pursuing, and what is most likely to block it?
CanUBuild checks the constraints on your exact address from national datasets — conservation areas, listed status, Article 4 directions, flood risk and tree preservation orders.
The side extension workflow captures the details that decide the outcome: width relative to the original house, height, storeys, materials and boundary position.
You see decided applications nearby so you can judge how your local planning authority treats side extensions on similar plots.
Sometimes. A single-storey side extension within the national height limit and no wider than half the original house can fall under permitted development — but only if your property still holds those rights and is not in a designated area.
Side extensions are more visible from the street and can affect the spacing between houses, so the national permitted development rules cap their width and height more tightly, and councils review their design impact more closely.
It can. Corner plots are more exposed to public view and side extensions on them often have a greater street-scene impact, making a full planning application more likely even where the basic dimensions look compliant.
Check the width of your original house, whether your permitted development rights are intact, and whether any conservation or Article 4 designation applies. An address-level check confirms these before you spend on drawings.
Search for the address, choose your project type, and get a planning feasibility answer based on permitted development rules, constraints, and local precedent data.