Calculation question-Renovation of dwelling with more than 25% of the surface of dwelling envelope?

I note on my BER Home Energy Upgrade Advisory Report the following:

"1. Major Renovation is defined in the Building Regulations Part L Technical Guidance Document and means the renovation of

a dwelling where more than 25 % of the surface of the dwelling envelope undergoes renovation. Where a dwelling

undergoes a major renovation, the energy performance of the whole dwelling should be improved to the cost optimal level

by achieving a B2 or by implementing the energy performance improvements as set out in the Building Regulations Part L

Technical Guidance Document."

How is 25 % of the surface of the dwelling envelope calculated? What constitutes the envelope? Roof, Walls, Doors, Windows?

Where could I get more information on this?

Thanks

PH