D1 BER: How much effort to get a B2+ rating?

ixoy

I want to improve the energy efficiency of the house, primarily with a view to making it warmer. It's currently D1-rated (largely detached) bungalow and is a single cavity block build with (old, not great) double glazed windows and a light layer of attic insulation. Vents are the old "hold in the wall". The flooring is suspended timber flooring.

If I go for a one-stop-shop, I'd need to achieve a B2 or higher. How much work (roughly) could be required? I know it's a bit of a tricky question and that the required assessment would tell me but before I fork out for all of that would appreciate it if someone has a very rough idea. I was thinking external insulation + new windows/doors + attic insulation but not sure that'd be enough. Going for the likes of an electric pump (to get to that B2+) would seem to be quite expensive and, from what I read, not necessarily going to save me a load of money long term for my build but it might be seen as "required" to get that B2+ rating.

sydthebeat

external insulation, new windows and doors, more efficient heating system and heating controls, and possibly some PV

if going for a heat pump system definitely get the floros replaced, suspended timber floor are terrible for energy efficiency

spaceHopper

If it's 30 ish years old expect to rewire it too