Most people hear “permitted development rights” and think it means they can skip planning permission entirely. Build what you want. No paperwork. No council involvement.
If only it were that simple.
The truth is, permitted development rights come with a long list of conditions that most homeowners never read. And the ones who skip the fine print? They usually find out the hard way.
At Extension Architecture, we have helped hundreds of London homeowners figure out what they can and can’t do under these rights. And we keep seeing the same mistakes. So let’s talk about the rules that catch people off guard.
Size Limits That Are Easier to Break Than You Think
Every type of permitted development work has specific size restrictions. And they are stricter than most people expect.
For a single storey rear extension on a detached house, the maximum depth is 4 metres. For semi detached and terraced homes, it’s 3 metres. Go even a few centimetres over and you are outside permitted development.
Height matters too. The maximum eaves height for a single storey extension is 3 metres. And if you are building within 2 metres of a boundary, the overall ridge height cant exceed 3 metres either.
These numbers sound simple enough on paper. But on site, things get tricky. Ground levels vary. Existing structures aren’t always measured correctly. And suddenly what you thought was compliant isn’t.
The Original House Rule That Catches Everyone
This is the one that surprises people the most.
All permitted development measurements are calculated from the original house. Not from the house as it stands today. The original house means the property as it was built, or as it stood on 1 July 1948, whichever is later.
So if the previous owner added a 3 metre rear extension fifteen years ago, that counts. Your remaining allowance is reduced by whatever they already used.
Many homeowners don’t even know about previous extensions when they buy a property. They assume the house has always looked the way it does now. Then they plan their own extension based on the full allowance and end up over the limit.
Always check the planning history of your property before you assume anything. The council keeps records. Your solicitor should have flagged any previous works during the purchase. If they didn’t, it’s worth looking into before you start spending.
Boundary Rules and Party Wall Complications
Building near a boundary is where things get complicated fast.
Under permitted development, side extensions can only be single storey and no wider than half the width of the original house. The height can’t exceed 4 metres. And you need to leave at least a small gap from the boundary in most cases.
But the rules and the practical reality don’t always line up. Even if your extension is technically within permitted development limits, you might still need a party wall agreement with your neighbour. That’s a separate legal process and it has nothing to do with planning permission.
Party wall matters can cause delays if your neighbour does not respond or decides to appoint their own surveyor. We have seen projects held up for weeks because nobody thought about the party wall until the builder was ready to start.
Plan for it early. It saves time and avoids awkward conversations later.
What Happens When You Get It Wrong
Getting permitted development wrong doesn’t just mean a telling off from the council.
If your extension is found to be outside permitted development and you don’t have planning permission, the council can issue an enforcement notice. That notice can require you to alter the work, reduce its size, or in some cases demolish it completely.
Even if the council doesn’t take action straight away, the problem doesn’t go away. It sits there waiting. And it usually surfaces when you try to sell. A buyer’s solicitor will check whether all building work was done lawfully. If you can’t prove it was, the sale stalls. Sometimes it falls through entirely.
We have worked with homeowners who had to apply for retrospective planning permission on work they genuinely believed was permitted development. Sometimes it gets approved. Sometimes it doesn’t. And that’s a stressful position to be in after you have already spent the money.
Loft Conversions Under Permitted Development
Loft conversions are one of the most popular home improvements in London. And many of them can be done under permitted development. But not all.
The volume allowance is 40 cubic metres for terraced houses and 50 cubic metres for detached and semi detached homes. That includes any previous loft work, not just what you are planning now.
Dormer windows on a front facing roof are not permitted development. Side facing dormers need to be obscure glazed and non opening below 1.7 metres. The materials should match the existing house. And the roof extension can’t be higher than the existing ridge.
If your property is in a conservation area, dormers and roof extensions that face a highway are also restricted. Some councils interpret “highway” broadly, so it’s worth checking even if you think your road doesn’t qualify.
A lot of homeowners assume a loft conversion is always permitted development. It often is. But the conditions are specific and easy to miss. One wrong measurement or one overlooked restriction and you need full planning permission instead.
Why Checking First Is Always Worth It
We know it feels like an extra hassle. You just want to get on with the project.
But five minutes checking the rules now can save you months of problems later. Look up your property’s planning history. Check whether you are in a conservation area. Find out if any Article 4 directions apply.
Or talk to someone who already knows. At Extension Architecture, we regularly conduct this kind of assessment for London homeowners. We check the rules, review your property, and give you a clear answer on what you can and can’t do.
London is complicated. Every borough has its own quirks. Permitted development rights are generous in some areas and heavily restricted in others. The only way to know for sure is to check before you build. Not after.
