Do the Part M access requirements apply to extensions to properties?
Guidance on this issue is in the latest 2004 edition of Part M. Broadly speaking, the answer is NO.
This is unusual for building regs: normally they apply to material alterations, just as much as to new builds. However, Part M is attempting to make homes easier for disabled people to visit and it therefore doesn't make any sense to apply these regs to just one part of the house.
There are a couple of exceptions.
• If the existing house is already Part M compliant (and it will be if it's been constructed in the past five years), then of course the new extension must meet the same standards.
• You can't make the house worse, from an access point of view. By way of example, you can't take out the only downstairs toilet.