We just had an extension built .. electrical works needed a certified installer, but the other building work including front door did not. However, the door itself needed to meet building regs - I think that anything with over 40% glass area needs double glazing, quite possibly in toughened glass. You may be getting that anyway with a upvc door (ours was wooden). Also, it's probably worth making sure you buy something with a locking system which meets insurers' minimum requirements / or gets you a discount on your insurance premiums. Cheers