Apologies to Helen, but you can go a lot further than 6' with an extension. It's called "permitted development."
Limits apply, but it's very common.
A rear extension should come under "permitted" unless it's a listed building.
My guess is they're going for an extension big enough to need PP.
In the meantime Smow, as someone else said, they can do a great deal to the interior that needs no permission.
What they will definitely need is "Building Regs Approval."
That's arranged with the Building Control Dept, and can begin within a few days of notification.
A great deal can be done without PP. Much more than people think. :o)