Many single-storey side extensions in England can go ahead without a planning application, because they fall under permitted development. The extension must stay within strict limits: one storey, no higher than 4 metres and no wider than half the width of the original house. Two-storey side extensions need planning permission, and so does any side extension on designated land such as a conservation area. If your house sits in Warrington, St Helens, Wigan or anywhere else in our patch, the rules below are the same, but your council’s local restrictions are what decide the final answer.
TL;DR
- A side extension is usually permitted development if it is single storey, no more than 4 metres high and no wider than half the original house.
- Anything two storeys, forward of the front wall, or facing a road at the side normally needs a planning application.
- On designated land (conservation areas, National Parks, AONBs and World Heritage Sites), side extensions always need planning permission.
- Permitted development does not replace Building Regulations or the Party Wall Act, which apply either way.
- A Lawful Development Certificate from your council gives you written proof that your plans are lawful before you build.
Table of Contents
- The short answer: when you do not need planning permission
- What counts as the “side” of the house
- When a side extension needs planning permission
- Side extensions that also reach the back: wraparounds
- Article 4 Directions and old planning conditions
- Can you be sure before you build? The Lawful Development Certificate
- The 45-degree rule and your neighbours
- Design points councils look at for side extensions
- Planning permission is not the only permission
- Typical timescales
- Common mistakes to avoid
- Our perspective
- Planning your side extension with us
- Different rules in Wales and Scotland
- FAQ
- Sources
The short answer: when you do not need planning permission
According to the Planning Portal’s single-storey extension guide, an extension or addition to a house is permitted development, meaning no planning application is needed, as long as certain limits and conditions are met. For a side extension, those limits are:
- it must be single storey
- it must be no higher than four metres
- it must be no wider than half the width of the original house
The same guide stresses that these allowances apply to houses, not to flats, maisonettes or other buildings. That matters in towns like Wigan and Warrington, where plenty of homeowners live in converted properties.
The word “original” is important. It means the house as it stood on 1 July 1948, or as it was first built if that was later. Earlier extensions, by you or by previous owners, count towards your allowance. Swindon Borough Council points out that the rules are applied to the proposed extension and any previous extensions together, the so-called total enlargement.
What counts as the “side” of the house
The side extension rule applies to anything built beyond a side wall of the original house. Say you have a semi-detached in Great Sankey or Penketh and want to build alongside the house where the driveway and garage are now. That is a side extension, and the half-width rule applies.
Here is a worked example. If the widest part of your original house is 8 metres, your side extension can be no wider than 4 metres. Measure the original house at its widest point, not the width of the plot or the gap you have available.
Extensions forward of the front wall
Nothing can extend forward of the principal elevation, which is usually the front wall facing the road. The Planning Portal also explains that an extension on a side elevation fronting a highway is not permitted development. Corner plots are the classic trap here. A house in Culcheth or Lowton on a corner may have a side wall that faces a road, which takes the side extension out of permitted development even though it feels like the side of the house.
Materials, windows and balconies
Permitted development comes with conditions as well as size limits. GOV.WALES, whose rules mirror England’s on this point, says materials should match the existing home as far as possible, and that verandas, raised platforms, roof terraces and balconies cannot form part of a permitted development extension. In England, upper-floor side windows have their own conditions, but since a single-storey extension has none, this mainly affects two-storey projects, which need planning permission anyway.
When a side extension needs planning permission
You will need to apply for householder planning permission when any of these apply:
- The extension is more than one storey. Swindon’s guide confirms that all side extensions of more than one storey require householder planning permission regardless of location.
- It is wider than half the original house, or taller than 4 metres.
- It sits forward of the front wall or beside a side wall that fronts a highway.
- Your home is on designated land. The Planning Portal states that on designated land, extensions on any side wall of the original house are not permitted development. Designated land includes conservation areas, National Parks, Areas of Outstanding Natural Beauty and World Heritage Sites.
- Your permitted development rights have been removed. More on this below.
- Your house is listed. Listed building consent is a separate process, and the Planning Portal notes other consents may be required in designated areas.
If you want a two-storey addition, our guide to two-storey extension permitted development in the North West walks through the specific rules.
Side extensions that also reach the back: wraparounds
A wraparound extension fills the space beside the house and continues across the back. GOV.UK’s technical guidance on householder permitted development says that where an extension fills the area between a side elevation and a rear wall, both sets of restrictions apply. In practice that means:
- it extends no more than 6 metres beyond the rear wall, or 8 metres for a detached house
- it is single storey and no higher than 4 metres
- its total width is no more than half the width of the house at its widest point
The half-width limit is the one that catches people out. A wraparound that spans the full back of the house will usually exceed it, so many of them go through a planning application instead. Joining onto an earlier extension can also cause problems, because the earlier addition uses up part of your allowance. We have written about this in planning a wraparound home extension and about extending an existing rear extension.
Article 4 Directions and old planning conditions
Even if your plans fit every size limit, your permitted development rights may have been taken away. There are two common ways this happens.
The first is an Article 4 Direction. Under the General Permitted Development Order 2015, the Secretary of State or the local planning authority can direct that permitted development does not apply in a specified area. These are most common in conservation areas and on streets of character.
The second is a condition on an earlier planning permission. Many estates built from the 1970s onwards had permitted development rights removed when the homes were approved, so the developer’s original consent can still restrict you today. Cheshire West and Chester Council’s guidance notes that you can check previous planning permissions on the council website, and most councils in Warrington, St Helens and Wigan offer a similar planning history search.
If you live in a newer development in Haydock, Rainhill or Great Sankey, checking this first can save you a lot of time.
Can you be sure before you build? The Lawful Development Certificate
GOV.UK’s technical guidance explains that to be certain a proposed development is lawful, you can apply to your local authority for a Lawful Development Certificate. It is not a planning permission. It is a formal decision that your proposal meets the permitted development rules, which can be useful when you sell, remortgage or when a neighbour questions the work. It costs less than a full planning application, and we explain how the choice works in our guide to permitted development versus planning permission.
The 45-degree rule and your neighbours
If you do need to apply, your council will look at how the extension affects neighbours. One test you will hear about is the 45-degree rule. Waverley Borough Council’s SPD describes it for side extensions: a 45-degree line drawn from the centre of a neighbour’s window, and if development crosses the line, the impact on daylight is likely to be unacceptable for that room. Barnsley Council applies the same idea to the nearest habitable room or kitchen window, while accepting that orientation, screening, roof design and changes in level can mean the test does not apply strictly.
The rule is a council design guideline, not part of the permitted development rules. Each council sets its own, so look at your own authority’s house extension guidance before drawing up plans.
Design points councils look at for side extensions
Councils often want a side extension to look subordinate to the original house. Cheshire West and Chester’s SPD says side extensions of similar height to the house will be supported where they are subordinate and do not exceed 50 per cent of the original width. Typical ways to achieve that are:
- setting the front wall back from the house’s front
- lowering the ridge height slightly
- matching brick, roof tiles and window proportions
- keeping a gap to the boundary where terracing would otherwise result
These are local policies, so the exact approach in St Helens or Wigan may differ, but the principle is common.
Planning permission is not the only permission
This is the point most homeowners miss. GOV.UK’s guidance is clear that permitted development does not remove the need for permissions under other regimes, including Building Regulations and the Party Wall Act.
- Building Regulations cover structure, foundations, fire safety, insulation, drainage and ventilation. A side extension nearly always needs approval, whether or not it needs planning permission.
- The Party Wall Act applies if you build on or near the boundary with a neighbour, which a side extension often does.
- Drainage is a real issue at the side of the house, where shared sewers and manholes are common.
The Planning Portal advises checking whether you need planning permission, building regulations approval or both, as these are separate applications.
Typical timescales
A rough sequence for a side extension, based on how these projects usually run:
- Design and drawings: a few weeks.
- Planning application, if needed: councils aim to decide householder applications within eight weeks, though it can run longer. Check your council’s current performance.
- Building Regulations: can run alongside planning.
- Construction: often a matter of weeks to a few months for a single-storey side extension, depending on size and complexity.
We give a fixed-price quotation once the scope is clear, so you know the cost before work starts.
Common mistakes to avoid
- Measuring the current house instead of the original. Earlier extensions reduce what you can add.
- Forgetting the corner plot rule. A side wall facing a road changes everything.
- Assuming a neighbour’s extension proves yours is fine. Their house may have different rights or history.
- Starting before checking Article 4 or old conditions.
- Skipping Building Regulations because planning was not needed.
- Ignoring the boundary. Building tight to the boundary can bring in the Party Wall Act and drainage questions.
Our perspective
After more than 35 years building extensions across Warrington, St Helens, Wigan and the surrounding areas, the question we hear most is whether a project needs planning permission. Our honest answer is that it depends on your house, your street and your plans, and the safest approach is to find out before you commit to a design.
The pattern we see is that a single-storey side extension on a standard semi or detached house often fits within permitted development, while corner plots, conservation areas, and homes where the width limit is tight need more care. Where a project is borderline, we would rather you apply for a Lawful Development Certificate or planning permission and have certainty than build and hope.
This is why we manage projects from the initial design stage through to completion. Our fixed-price quotations mean you know the cost before work begins, and as a member of the Guild of Master Craftsmen we take Building Regulations and neighbour matters as seriously as the build itself. When the paperwork is right from the start, the building goes smoothly.
Gareth
Planning your side extension with us
If you are thinking about a side extension, kitchen extension or garage conversion in Warrington, St Helens, Wigan, Rainhill, Culcheth, Haydock or nearby, we can look at your plans, explain which route is likely to apply, and manage the work from design to completion.
See our house extensions services, or get in touch for a conversation about your project. If you are weighing up the wider process, our guide to how home extension planning works is a good place to start.
Different rules in Wales and Scotland
The rules above are for England. If you are reading from outside it, they differ. In Wales, single-storey side extensions must not exceed 4 metres in height and the widest part of the resulting house cannot exceed the original by more than 50 per cent, according to GOV.WALES. In Scotland, Planning Circular 1/2024 says single-storey extension rights do not apply in conservation areas. Northern Ireland has its own system again. We work in North West England, so our advice is based on the English rules.
FAQ
Do I need planning permission for a single-storey side extension?
Often not. It can be permitted development if it is no higher than 4 metres and no wider than half the original house, and does not sit forward of the front wall or beside a side wall facing a road. It always needs planning permission on designated land.
Do I need planning permission for a two-storey side extension?
Yes. Councils such as Swindon confirm that all side extensions of more than one storey need householder planning permission wherever they are. Building Regulations approval will also be needed.
How do I find out if my permitted development rights have been removed?
Contact your local planning authority and check your property’s planning history on the council website. Rights may have been removed by an Article 4 Direction or by a condition on an earlier planning permission.
Do I still need Building Regulations if I do not need planning permission?
Yes. GOV.UK guidance states that permitted development does not remove the need for Building Regulations or Party Wall Act consents. They are separate from planning.
Can I get proof that my side extension is permitted development?
Yes. You can apply to your local authority for a Lawful Development Certificate, which confirms in writing that your proposal is lawful and does not need a planning application.
Sources
- Planning Portal: Single-storey extension mini guide
- GOV.UK: Permitted development rights for householders, technical guidance
- Swindon Borough Council: Your guide to planning
- GOV.WALES: Planning permission for extensions
- GPDO 2015, Article 4 wording
- Cheshire West and Chester Council: House Extensions SPD
- Waverley Borough Council: Residential Extensions SPD
- Barnsley Council: House Extensions SPD
- Scottish Government: Planning Circular 1/2024
