Your oddly chosen description of the 3 metre back issue and “all on the ground floor” intrigues me.
Have you recently opened a window on an upper floor within 3 metres of the boundary without permission?
If not, carry on; –
If you build a flat roof extension with parapet walls onto a bungalow with a pitched roof and eaves it may not be considered exempt – eaves height is just one issue, materials are another.
Also building to the rear on the shared boundary with a semi-D is a whole other world of uncertainty and grief at the moment.
Suggestion; –
Draw it up and talk to the planner.
Better still seek a Section 5 Declaration.
Although even the planners can get it wrong.
ONQ.