You will need a combination of manufacturer's instructions, and consideration of ergonomics for maintenance tasks. BS EN 547 series can help with this.

The minimum dimensions in Section 729 of BS 7671 apply where certain specific protective measures restricted to skilled and instructed persons are used (not always relevant in the UK). But manufacturers may recommend increasing these.

More space may be required with DC equipment, particularly batteries or equipment connected to batteries, if there are arc flash hazards.

Also there are BS 7671 general requirements (e.g. 132.12) for accessibility of electrical equipment for operation, inspection, testing, maintenance, repair and so on - no specific numbers, but a bit together with a bit of common sense inform maintenance/safety procedures etc. (E.g. if there was a policy of taking IR pictures of DBs under load with their covers off,  and H&S policy required two people to be present, then there would have to be sufficient space to do that safely).

Also building regs might have a say where the occupants might reasonably require access (part M) - probably more likely for a domestic (AD M suggests specific heights).

   - Andy.

I always considered up against the ceiling, over the toilet in a pokey cloakroom to be the optimal position!

I like it (CU/DB) under the stairs at the tightest spot it will fit into for a domestic house.