By Rob Allison, founder of Thames Design & Build
Some extensions in Surrey don’t need full planning permission. Something called Permitted Development covers this — it lets homeowners extend within set limits, with no planning application at all. But it’s easy to assume your project qualifies when it doesn’t. The rules that decide which side of that line you’re on catch people out more often than you’d think. That includes us, in our early years.
Here’s what this article covers. First, what Permitted Development actually includes. Second, what a Lawful Development Certificate is, and why we always recommend one. And two specific ways extensions and loft conversions can fall outside Permitted Development, in ways almost nobody checks for until it’s too late.
In short: many single-storey rear extensions and loft conversions in Thames Ditton, Esher, Cobham and the rest of Surrey can be built under Permitted Development without planning permission, provided they stay within set size and volume limits. But two things regularly catch people out: an existing dogleg in the rear wall, and roof volume already used by a previous extension. Getting a Lawful Development Certificate before you build is the only way to know for certain rather than assume.
What is Permitted Development?
Permitted Development (PD) rights let you extend your home without a full planning application. You just need to stay within specific size and height limits set out in national planning law. For a single storey rear extension, that typically means up to 3 metres for a terraced or semi-detached house. Detached houses get a bit more: up to 4 metres, under the standard rules. Larger extensions — up to 6 or 8 metres respectively — are possible too, under a separate Prior Approval process that involves notifying your neighbours. Slightly counterintuitively, Prior Approval has a shorter statutory timeframe than either an LDC or a full householder application — 42 days against the usual 56. That’s because the council is only assessing neighbour impact, not checking the scheme against every planning rule in detail.
These rights can also be restricted or removed entirely — in conservation areas, on listed buildings, or where
a previous planning condition has taken them away. We check this as a matter of course during your first site
visit. For the full detail, the government’s own permitted development rights guidance for householders sets
out the rules in full.
What is a Lawful Development Certificate, and do you need one?
A Lawful Development Certificate (LDC) is a formal confirmation from your council that your scheme genuinely qualifies as Permitted Development. It’s optional — you’re not legally required to have one before building under PD.
We always recommend getting one anyway. Without it, you’re relying on your own interpretation of the rules holding up, with nothing in writing if a neighbour complains. That matters too if you come to sell the house, or if a future buyer’s solicitor asks for proof the work was lawful. An LDC costs a small planning fee and has the same 8-week statutory timeframe as a full householder planning application. But it removes that risk entirely. It’s cheap insurance for something that otherwise sits as a permanent question mark over the property. It’s also exactly the kind of thing that’s easier to get right when the same team handles both design and planning. Split that across separate people, and this sort of detail is more likely to slip through.
Where extensions catch people out: the dogleg
Most people assume a rear extension is simply a rear extension. But if the existing back wall of the house isn’t flat, that assumption can break down. A step or dogleg in the wall is common on semi-detached and terraced houses. It happens where one side has already been extended, or the original layout wasn’t symmetrical to begin with. Extend across that dogleg, and you’re technically adding to the side of the house as well as the rear — not just the rear.
That matters because Permitted Development limits side extensions to half the width of the original house. An extension that would be perfectly fine as a straightforward rear extension can suddenly fail to qualify. All it takes is an existing kink in the wall that has nothing to do with the new design. That kink alone can force the whole extension to satisfy the side extension width limit too.
Where loft conversions catch people out: volume already used
Permitted Development for loft conversions works on a volume allowance, not a floor area. You’re allowed to add a set volume of roof space — measured in cubic metres — before you need planning permission. What catches people out is that a change of owner, or the passage of time, doesn’t reset this allowance. It stays tied to the house, and any previous roof extension already counts against it.
Say a house already has a first floor side extension with a pitched roof. That roof space has already used up part of the volume allowance — even if it was built years before the current owners moved in. It doesn’t matter if nobody living there today thinks of it as “a loft conversion.” A new loft conversion then has less headroom under the allowance than it would on an equivalent house that’s never been touched. It can tip over the limit in a way that isn’t obvious, until someone actually adds it up.
We learned those the hard way
Both of these caught us out over ten years ago, in TDB’s early days. We applied for Lawful Development Certificates on two separate projects, confident both qualified as Permitted Development. One hit the dogleg issue, the other hit the loft volume issue — and the council refused both. In each case, we had to go back and start again with a full householder planning application instead. That effectively doubled the time spent on the application stage — time neither the clients nor we wanted to lose.
Neither was a pleasant conversation to have with the clients involved. But that’s exactly why we now check both of these specifically, on every single scheme. We do it before we ever tell a client their project can go ahead under Permitted Development. It’s a small thing that’s easy to miss and expensive to get wrong. We’d rather have learned it once, early, than risk it happening to a client today.
What this means for your project
The real lesson from both of these isn’t the dogleg rule or the loft volume allowance specifically. It’s that you shouldn’t assume your project qualifies as Permitted Development, just because it looks like a fairly standard extension or loft conversion. The size and shape of the finished scheme can look completely ordinary. But it can still fall foul of a technicality that has nothing to do with how big or small it looks from the outside.
We check for exactly this kind of thing as standard, during our initial site visit. That means the dogleg rule, the loft volume allowance, conservation area status, Article 4 directions, and anything else that could affect your Permitted Development rights. It’s the same site visit where we start giving you budget cost advice for your project. That way you get a clear picture of what’s achievable and what it’s likely to cost, together, rather than as two separate conversations. If you’re not sure where your project stands, get in touch and we’ll give you a straight answer.
Quick answers
Do I need planning permission for a rear extension in Surrey?
Often not, if it stays within Permitted Development size limits. But an irregular or staggered rear wall can push it into side extension rules, which are stricter.
Does a loft conversion always fall under Permitted Development?
Not automatically. The volume allowance stays tied to the house, not the current owner. So a previous roof extension can reduce how much room you have left.
Is a Lawful Development Certificate worth getting if it’s optional?
Yes. It’s the only way to have written confirmation from the council that your scheme is lawful. That matters if a neighbour objects, or if you sell the house later.