Key takeaways
- Many single-storey rear extensions fall under permitted development and need no planning application, but size and height limits still apply.
- A detached house can extend up to 4 metres to the rear under standard permitted development, and a semi-detached or terraced house up to 3 metres.
- A householder planning application in England costs £528 from 1 April 2025, with a statutory decision target of 8 weeks.
- Conservation areas and listed buildings remove many permitted development rights, so always check your property's status before you build.
- Submit your application through the national Planning Portal; West Northamptonshire Council decides applications for Northampton.
Many house extensions do not need planning permission at all. Single-storey rear extensions, side returns and some loft projects often fall under permitted development rights, which let you build within fixed limits without a formal application. Whether your project qualifies comes down to the type and size of the extension, where you live, and whether your home has any special protections. This guide walks you through when planning permission for an extension is required, the permitted development limits that apply, how to apply to your council in Northamptonshire, and what to expect on cost and timescale.
When an extension needs planning permission
Permitted development rights allow certain building work without a planning application, as long as the project stays inside government limits. Step outside those limits and you move into territory where full planning permission is needed.
As a general rule, you will need to apply for planning permission when your extension:
- Covers more than half the land around the original house
- Extends beyond the front of the house facing a road
- Is taller than the highest part of the existing roof
- Sits on a property where permitted development rights have been removed
Permitted development is usually the faster, cheaper route because it avoids a formal decision. That said, assuming your project qualifies and building anyway is a costly mistake. If the work turns out to need permission, the council can require you to apply after the fact or, in the worst case, undo the work. When there is any doubt, a Lawful Development Certificate from the council confirms in writing that your plans are permitted development, which is useful evidence when you later sell the house.
For a full picture of the budget before you reach the planning stage, see our guide on how much a house extension costs in 2026.
Size and height limits under permitted development rights
The limits depend on the type of house and where the extension sits. For single-storey rear extensions, the standard depth allowances are:
- Detached houses: up to 4 metres beyond the original rear wall
- Semi-detached and terraced houses: up to 3 metres beyond the original rear wall
These figures come from the Planning Portal's guidance on extensions, the official source for planning in England. A single-storey rear extension must also be no more than 4 metres high overall, and where it comes within 2 metres of a boundary, the eaves height cannot exceed 3 metres.
There is a route to go further. The Larger Home Extension scheme, made permanent in 2019, lets you build a single-storey rear extension up to 8 metres on a detached house, or up to 6 metres on other houses, through a prior approval process rather than a full application. You submit details to the council, which consults your neighbours for 21 days. If a neighbour objects, the council assesses the impact on their amenity before deciding. This larger allowance does not apply on designated land such as conservation areas.
How to apply to your local council in Northamptonshire
If your extension needs permission, applications are made through the national Planning Portal, the government's online service. West Northamptonshire Council is the local planning authority for Northampton and the surrounding area, so it is the body that will decide your application.
The typical steps are:
- Check whether you need permission. Use the Planning Portal's interactive house tool, or ask the council for pre-application advice.
- Prepare your drawings. You will need existing and proposed plans and elevations, drawn to scale, plus a site location plan.
- Submit and pay online. Applications and fees go through the Planning Portal, which forwards your submission to West Northamptonshire Council.
- Validation. The council checks your application is complete. The decision clock starts from the validation date, not the submission date.
- Consultation and decision. Neighbours and consultees are given around 21 days to comment before the council reaches a decision.
Most householder applications are decided by planning officers under delegated powers rather than by a committee, which keeps straightforward cases moving. Pre-application advice is worth the small fee when your extension sits close to a boundary or near a protected feature, because it flags problems before you commit to full drawings.
Dealing with conservation areas and listed buildings
Special protections change the rules considerably, and this is where many homeowners get caught out.
In a conservation area, which counts as designated land, permitted development is tightly restricted. Side extensions and two-storey extensions are not permitted development, the larger 8 metre and 6 metre rear allowances do not apply, and cladding the exterior in stone, render, timber, plastic or tiles needs permission. Some conservation areas also carry an Article 4 Direction that removes further rights, so even minor changes may need an application.
If your home is a listed building, you will usually need listed building consent for any work that affects its character, inside or out, on top of any planning permission. This protection can extend to outbuildings and structures within the grounds. The penalties for unauthorised work to a listed building are serious, so professional advice is essential before you start.
Before you plan an extension on an older or characterful property, check the council's records to confirm whether your home is listed or sits within a conservation area. That single check can save months of wasted design work.
Timescales and typical application costs
The fee for a householder planning application in England is £528 from 1 April 2025, covering the enlargement, improvement or alteration of a single home. That figure, confirmed in the 2025 fee regulations reported by planning consultancy Lichfields, rose from £258, and fees are now reviewed each April in line with inflation.
On timing, the statutory target for a householder application is 8 weeks from validation. In practice, many straightforward cases are decided within that window, though applications that attract objections or need extra information can run longer. The neighbour consultation period of around 21 days sits inside that eight-week clock.
It is worth budgeting for more than the council fee alone. Scaled drawings from an architect or technician, a structural engineer where needed, and building regulations fees all add to the total. Building regulations approval is separate from planning permission and applies even when your extension is permitted development, so factor it in regardless of which route you take.
How AAS Group supports you through the planning process
AAS Group Construction works with homeowners and developers across Northampton and the wider Northamptonshire area, and the planning stage is where a good contractor earns its keep. We help you work out early whether your extension is likely to be permitted development or needs a full application, which avoids expensive surprises once work starts.
Our involvement typically includes:
- Advising on realistic design options that stay within permitted development limits where you want to avoid an application
- Coordinating with architects and structural engineers on the drawings a planning submission requires
- Flagging conservation area or listed building constraints before you invest in detailed designs
- Managing the build to meet both planning conditions and building regulations
Getting the planning route right at the start keeps the whole project on track. If you are weighing up an extension on your Northampton home, the next step is simple: confirm your property's planning status with West Northamptonshire Council, then talk to a contractor about a design that fits within the rules. Contact AAS Group to discuss your extension and we will help you map out the planning path before you spend a penny on drawings.
Frequently asked questions
Do I always need planning permission for a house extension?
No. Many extensions fall under permitted development rights and need no application, provided they stay within set size and height limits. Larger or more complex projects usually need full planning permission.
How much does a householder planning application cost in England?
The fee for a householder application is £528 from 1 April 2025. This covers enlargement, improvement or alteration of a single dwelling.
How long does a planning decision take?
The statutory target for a householder application is 8 weeks from the date your application is validated. Neighbours are usually consulted for 21 days during that period.
What if my home is in a conservation area?
Conservation areas remove many permitted development rights. Side extensions, two-storey extensions and certain cladding usually need full planning permission, so check with the council first.
Do I still need building regulations approval if my extension is permitted development?
Yes. Planning permission and building regulations are separate. Your extension must meet building regulations for structure, insulation and safety even when no planning application is required.
