Do I Need Planning Permission for an Extension?

Permitted development: when you can skip planning permission

Permitted development rights let you extend a house without a planning application, provided the design stays within set limits. They apply to houses only, not flats or maisonettes, and only if a previous owner has not already used them up. Plenty of homes on Yeovil's older estates have had garages converted or kitchens extended over the years, so it is worth checking the property's planning history on the Somerset Council website before you assume you have full rights left.

The headline limits for a single storey rear extension are 4 metres deep for a detached house and 3 metres for a semi or terrace, with a maximum height of 4 metres. Under the larger home extensions scheme you can go to 8 metres and 6 metres respectively, but that route requires a prior approval application so the council can consult your neighbours first. Side extensions must be single storey, no wider than half the original house, and no taller than 4 metres.

• Rear extension: up to 4m deep (detached) or 3m (semi or terrace) without any application
• Larger extensions up to 8m or 6m need prior approval, which takes around 6 weeks
• Extensions cannot cover more than half the land around the original house
• Materials should be similar in appearance to the existing house
• No extension forward of the front wall facing a road

When you will need a planning application

Two storey extensions almost always need permission unless they are modest rear additions meeting strict conditions, including being at least 7 metres from the rear boundary. You will also need an application if your design breaches any permitted development limit, if you want cladding or render that differs noticeably from the house, or if you live in a flat.

Location matters too. Parts of Yeovil town centre sit within conservation areas, and villages such as East Coker, Montacute and Stoford have conservation areas and plenty of listed buildings. In those settings permitted development rights are reduced, and side extensions in particular usually need consent. Listed buildings need listed building consent for almost any alteration, inside or out, regardless of size. If you are anywhere near the Ham stone terraces that make this part of Somerset distinctive, check before you design anything.

Costs, timescales and the paperwork worth having

A householder planning application in England currently costs £528, and Somerset Council aims to decide within 8 weeks, though busy periods can stretch that. Prior approval for a larger extension costs £120 and runs on a 42 day clock. On top of the fee you will typically pay £600 to £1,500 for measured drawings from an architectural designer, depending on the complexity of the scheme.

Even when no permission is needed, we usually recommend applying for a lawful development certificate, which costs half the planning fee. It is formal confirmation from the council that your extension was legal, and solicitors increasingly ask for it when you sell. It is far cheaper to get one before building than to prove compliance retrospectively years later.

One thing catches people out constantly: building regulations approval is separate from planning and is needed for virtually every extension, covering structure, insulation, drainage and electrics. Your builder should be working to building regs whether or not planning permission applies, with inspections booked at key stages.

Practical steps before you commit to a design

Start by checking your property's planning history and any constraints through the Somerset Council planning portal, then get a designer or builder to confirm whether your ideas fit within permitted development. If the scheme is borderline, a pre application enquiry to the council costs less than a refused application and redesign.

Talk to your neighbours early, especially for anything near a shared boundary. You may also need a party wall agreement under separate legislation if you are building within 3 metres of a neighbour's foundations, which is common on Yeovil's semi detached streets. None of this is complicated once you know the sequence, and a good local builder will have been through it dozens of times and can point you to designers who know how the council interprets the rules.