How To Build a Two Storey House Extension WITHOUT PLANNING PERMISSION

A two storey house extension in the UK

Do you own a home in the UK but it isn't large enough? Do you want to build a two-storey house extension without having to apply for planning permission? Believe it or not, this is possible but I don't think its a good idea. Let me explain. 

Permitted development rules were updated in 2024 to include two-story extensions. In this post, I'm going to show you how this works and why I think both permitted development and two-storey extensions can cause serious issues. If they are not managed properly, they could ruin your home. 

What is Permitted Development?

Permitted development allows homeowners in the UK to extend their property without the need to apply for planning permission, but the permitted development system sets strict limits on the size of any extension. Not every home has permitted development rights. If you live in a flat or maisonette or your home is listed, then your property does not have permitted development rights and any extension to your home will need planning permission. If your home is in a conservation area, national park, area of outstanding natural beauty or a world heritage site, its permitted development rights can be limited or even removed entirely by your local authority. 

Always check with the local planning department; they publish maps of relevant areas, showing what permitted development rights homes have, if any. 

Using permitted development to build two-storey house extensions

Permitted development started 10 years ago and, due to its popularity, the government have increased the size and type of buildings that can benefit from permitted development rights. Several changes were introduced in 2024. One of those new changes is the ability for some homes to build a two-story extension under permitted development. But, as with everything in permitted development, there are limitations.

• First off, the existing house must have at least two stories. You won't be allowed to build a two-storey extension on a one-storey house. 

• The two-storey extension can only be at the rear of the house, not at the side. 

• The extension can't take up more than 50% of the area around your home. Be aware that this calculation can be complex. The measurements take outbuildings and previous extensions into account and if you want to build an extension this large, you should take professional advice first.

• The two-storey extension cannot be any more than three meters out from the existing rear wall of the house and there must be at least 7 meters between the new extension and the rear boundary of the garden. This figure varies to 10m in Scotland and 10.5m in Wales 

• The extension must use materials that match the existing house.

• The slope of the extension roof must match the existing house roof, and the extension's eaves and ridge cannot be taller than the existing house's eaves and ridge. 

• The design cannot have any balconies on the upper floor. 

• If any extension comes within two metres of any boundary, then the eaves cannot be taller than three metres. This makes it impossible to build a two-storey extension unless it is more than two meters away from any boundary. 

• And any widows on the side walls of the new extension must have translucent glazing and they can't be openable, unless the openable part is more than 1.7m above the internal floor.

Why permitted development can be a trap for homeowners

There are two ways that permitted development can be a trap for homeowners;

First, while permitted development is very useful for homeowners, keep in mind that it was introduced to make life easier for the planners. Permitted development removed a large number of small applications from the system, freeing up time for the planners to focus on larger applications. Every week I give advice to homeowners from all corners of the UK during my online consultation service.

No two clients are the same but many of them fear the planning system. I have lost count of the number of people who want to use permitted development because they worry unnecessarily about lodging a planning application. They see permitted development as a cheat code that will get them an extension which wouldn’t otherwise be allowed, and that is the wrong way to look at it. 

If a house has permitted development rights, it may have a reasonable chance of getting planning approval to build a larger extension than permitted development will allow. Always start by looking at what you need. If a larger extension is affordable and will give you a better home, do not restrict yourself by creating a smaller extension just because that is what permitted development allows. If you choose to limit the size of your house extension just to avoid making a planning application, this is a false economy.

The second way that permitted development can trap homeowners is when they come to sell a house that has been extended under permitted development. When the system was introduced, it did not require homeowners to interact with the planning system, the whole point was to reduce the workload for planning officers. 

The planning system does not require homeowners to make an application to confirm they have permitted development rights. But that has not stopped mortgage providers from asking for proof that extensions were designed under permitted development. Mortgage lenders regularly for a Lawful Development Certificate as evidence that extensions are permitted development. 

While you could apply for an LDC retrospectively, it may not solve the problem because the planning legislation gives local authorities the power to reduce or remove permitted development rights. What might have been permitted development when you built that extension may no longer be the case. If you apply for an LDC retrospectively, the planners will only consider the rules in force on the day you make the application, not the rules as they were back when you built that extension. In this situation, you could be refused an LDC and then have to apply for retrospective planning approval, with the risk that could be refused and the extension would have to be demolished. 

I would advise anyone considering using permitted development to apply for a Lawful Development Certificate before building anything, even if the planning system does not require one. The planners will process the application and confirm in writing that your proposed design has permitted development. This can save a lot of hassle if you sell the house later on. 

Why two-storey house extensions are not the answer 

Two-storey extensions often sound like an ideal way to increase the size of a house, but they can create more problems than they solve. If you want to build on top of an existing one-storey structure, such as a garage, it's almost certain that the foundations of that structure were not designed to carry the extra load of another storey on top.

I wrote a post about that problem last year, you should check it out. 

In order to get that two-story extension, you may end up having to dismantle the garage, dig out its foundations and start from scratch. Any savings you expected to get by using an existing structure could turn into additional costs to remove that structure. 

In my experience, people want a two-storey extension because they need more bedrooms. Most two-storey homes in the UK are designed with living space on the ground floor and bedrooms upstairs, so this makes sense. The problems start when trying to connect the new room to the existing hall or stairs. That could involve creating a new corridor through an existing bedroom to access the new bedroom. This, too, could be a false economy. 

In my own practice, I have talked people out of building a two-storey extension and instead creating a larger one-story extension, then converting an existing room on the ground floor into a new bedroom. This usually involves applying for planning permission but this is seldom an issue.

Summary 

After hearing my thoughts on permitted development, do you still think it's right for your home? And if you are thinking about extending a home anywhere in the UK, you can get my advice by booking an online consultation. Please read the terms and conditions before you book.