It sound like you're free of planning constraints - however it may be
worth making a written enquiry to your local planning dept so that you
have written confirmation that it's not necessary.

I'm not sure about the exception to Building Control inspections for
buildings of less than 30m2 - however don't see BC as an adversary -
generally they're interested in buildings being safe and are more of a
help than a hindrance. Whether you need your structure inspected or
not - you're still required to conform to the regs.

The only other potential issue is if you live in a Conservation Area.
Again informal enquiries to planning control will answer that.