Permitted development question

[S1MMA]

Hi all, I'm considering buying a property to renovate so just getting my head around PD rules and regs. This will be in North London, and not in a conservation area.

I’m looking to buy a detached house, so understand that the maximum single story rear extention is currently 8m (subject to PD conditions) and after neighbor consultation, and 3m for a double story extension.

My question is, could you do an 8m single story rear ground extension, and also a 3m first floor rear extension? Basically combine both sections of the rules? Or can you only do one or the other?

I’ve not seen this scenario anywhere so interested to see if it’s possible?

Also if you are undertaking a rear extension, is there anything stopping you also doing a side extension and even roof extension at the same time, say a rear dormer?

Thanks!