Here in west london they're a bit oermissive, so instead of straight away refuse your planning application, they advise on what would be required for them to look at the plans more favourably.
In fact, a lot of time they just approve tbe application subject to conditions and then ask for revised plans.
Now, IF you need planning permission, then you could submit the same plans and see what happens.
OR, submit a pre-application.
Bear in mind though, I never liked these pre-application business because it's looked at by junior officers who like to make a name for themselves and find lots of faults.
Before anyone says anything, this is my experience and in other places it may well be totally different.
Example: a customer against my advice decided to submit planning permission application for a 4.5 metre single storey rear extension which was permitted development.
They came back and said only 3 metre was allowed.
This is totally incorrect, or at least it was incorrect at the time because of the 6 metre rule.
In any case, we ended up building a tiny 3 metre extension for the same price: good for us, not so good for the stubborn landlord.
You could also apply for permitted development confirmation (I'm sure it's got a more posh name), this will tell you if you need planning permission or not and it's not expensive.