Permitted development rules for loft conversion — do I need permission?

I'm looking to do a loft conversion with a dormer window at the back. The property is in a standard residential street, not a conservation area.

From what I can tell, PD rights allow dormers as long as they don't extend beyond the plane of the existing roof slope on the principal elevation. But I'm confused about whether the volume limit (40m3 for terraced, 50m3 for detached) includes the existing loft space or just the new addition.

Anyone gone through this recently? Did you just do it under PD or play it safe and apply anyway?

I did exactly this last year. The volume limit is for the new space only, not the existing loft. So if your current loft is 20m3 and your dormer adds 25m3, you're fine on a terraced house (under 40m3).

That said, I'd still recommend a Lawful Development Certificate. Costs about £100 and gives you written proof it's lawful. Much better than trying to sell the house later and the buyer's solicitor asking awkward questions.

Be careful with the "principal elevation" rule. If your house faces the street and the dormer is at the back, you're fine. But if your house is on a corner and the dormer is visible from the street-facing side, some councils count that as the principal elevation.

Best to check your council's own PD guidance — most have a supplementary planning document that clarifies their interpretation.

We went the LDC route and it took 6 weeks. The council didn't even visit — just checked the drawings against the rules and issued the certificate. Worth every penny for peace of mind.

One thing they did flag: our builder had drawn the dormer 10cm too high in the initial plans. Easy fix, but would have been an expensive mistake if we'd built it and then got caught.

Thanks all, really useful. I think I'll apply for the LDC and get the drawings checked properly. Better safe than sorry.