if your flat entrance door is a 1/2 hour fire door with a self closer and smoke seal intumesent strips this would be reasonable. You could ask your neighbour to pay for the up grade.
You need building regulation approval, find yourself a building professional such as a Building Surveyor or Architect who will produce drawings and specifications for your builder to build to.
Sounds like an exempt building as far as building regs apply, check it out on the planning portal website. So you can put your foundations as deep or as shallow as you like.
