The short answer
It is not a legal requirement, but for a modular extension in London it is close to essential. It is the council's written confirmation that the extension is permitted development, it costs £274, and it protects you before the factory builds anything and when you come to sell.
Permitted development lets you build some extensions without planning permission. Nobody checks your judgement that the extension qualifies, though, until someone complains or a buyer’s solicitor asks. A lawful development certificate is how you get the council to check it first.
What the certificate does
You send the council drawings of the proposed extension and ask it to confirm that it would be lawful. The council assesses it against the permitted development rules: depth, height, eaves, distance to boundaries, materials, and whether the house still has its rights. If it qualifies, you get a certificate that says so.
It is a legal test, not a planning judgement. Neighbours are not consulted, and design is not assessed. The answer is yes or no.
Cost and timing
The fee for a proposed development is half the normal planning fee: £274 in England from 1 April 2026, against £548 for a householder application. Councils aim to decide within 8 weeks, and many are quicker for a simple rear extension.
Why it matters more in London
Rights are often already gone. Flats, maisonettes, conservation areas, Article 4 directions and conditions on old permissions all remove or limit permitted development. Many London houses are affected by at least one, and it is not always obvious.
The original house is hard to read. Terraces have often been extended before. What counts as the original rear wall decides how far you can go, and a 1970s infill or an old conservatory can change the answer.
Side returns sit in a grey area. Councils usually treat a side return infill as a side extension, which changes the limits. A certificate settles which rules apply.
Why it matters for a modular extension
A traditional builder can redesign a wall halfway through if a council officer visits. A modular extension is made in a factory to fixed dimensions and lifted in on one booked day. If the council later says it should have been 50 centimetres shorter, there is no cheap fix. Get the certificate before the factory order is placed.
When you sell
Buyers’ solicitors ask for planning evidence for every extension. Without a certificate you may be asked to apply after the event or take out indemnity insurance, both of which slow the sale.
When you do not need one
If the extension needs planning permission anyway, the permission is your evidence. And if it needs prior approval under the neighbour consultation scheme, that approval covers the depth question, though not every other condition.
For prices and builders, start with our guide to modular extensions London homeowners use, or price your size on the cost calculator.