Question ? What is the situation in the UK - is it all the rooms. Reference is made to living space but do not know if you live in the bathroom or loft conversion

In the UK we don't use the measurement in the headline.  The advertised feature here is always like "3 bedroom house" instead of "100m2 house". 

Here the measurements will be somewhere within the advert, but each and every room will be mentioned like 5x4m. You can then of course work it out.

The confusing bit for me is I may see an advert for a flat with 5 pieces @ 100m2 . Then it says 3 bedroom, 1 living room, 1 kitchen, 1 shower room, 1 toilet, 1 cellar .. Which adds up to more than 5 - So which rooms are measured to make that 100m2?

Yeah what's a half room  :)

In France, it includes bathroom/kitchen, but not stairs/elevators. It even includes the space taken by insulation along the exterior walls. (So as not to discourage owners from adding more insulation.)

For area: you measure nett area of apartment including all rooms, corridors etc, also stairs (but not space below stairs). Rooms or part of the rooms with too low ceiling (i.e. under the roof) should not be calculated in. Walls, installations (chimney etc), openings for door are also not taken in account. Not sure how is it with cellar.

Number of rooms: you take in account all main rooms: living (and dinning) room and all bedrooms, the half room should be a separated kitchen. Bathroom, toilet or storage rooms should not be taken in account.

There is a strict standard for newly build apartment how to calculate this.

In attic rooms you count everything where the celing is over 1.5m high!