Rod Carrol wrote
Question: A friend says I may have difficulty in getting planning
permission for this because I already have a loft conversion and may
have used up my 'quota'.

Does this sound right?

No. I think the "quota" he's on about is the amount it is allowed to enlarge a
property without needing to apply for formal Planning Permission. There are
rules, but for terraced houses this is 50 cubic metres or 10% of the volume of
the original house (whichever is greater), with an upper limit of 115 cubic
metres. There's nothing to say you can't extend the house by more than this,
only that you need to apply for formal Planning Approval to do so. In any case,
I'm pretty sure a conversion of an existing loft space inside the original
structure doesn't use up any of this allowance (except maybe the dormers).

The legislation relating to this is The Town & Country Planning General
Development Order 1988, known as the "GDO". http://tinyurl.com/2trc7
If you build under this allowance, make sure you get a "Certificate of Lawful
Development" from the Planners. This will avoid any nasty questions from the
buyers Solicitors when you eventually sell the house.

I agree with Brian - go and see the Planners.

Peter