If I am going to re-build my loft hatch, do I have to conform to any new
building regulations or can I do what ever I want?

What I want to do is fit a frame within the existing opening using
wooden strips. The hatch will be cut from 18mm plywood and will fit
within the opening up against the newly fitted frame. The ceiling side
of the hatch will be flush with the ceiling. I will have self adhesive
foam strips stuck to the loft side of the hatch which will be compressed
against the frame, hopefully forming a moisture and draught seal. I will
then fix some celotex to the loft side of the hatch for insulation.

Any opinions?

Thanks.