Building control vs planning permission — which do I need first?

Complete beginner question I'm embarrassed to ask. We're doing a single-storey rear extension (4m deep, under PD).

Do I need to submit a building control application BEFORE starting work, or can I just crack on and notify them later? And is building control completely separate from planning, or does one trigger the other?

Our builder says "just start and we'll sort building control afterwards" but that feels wrong. What's the actual legal position?

You need to submit a Building Notice or Full Plans application BEFORE starting work. Doing it afterwards is possible but much more expensive and stressful because the inspector can make you expose work that's already covered up.

Your builder is being lazy. Any reputable builder knows this. If they're suggesting you skip it, that's a massive red flag.

Agreed. We used a Building Notice for our extension and the inspector visited 3 times during construction. Each visit was about 30 minutes. No drama, no delays. Cost was about £300 total.

The "full plans" route is more paperwork upfront but gives you certainty before you start. Building Notice is simpler but you need to trust that your builder knows what they're doing.

Thanks both. I'll tell our builder we're doing it properly and see what they say. Might be time to find a new builder...