You are not "usually always better off" - what a ridiculous sweeping generalisation. Have you personally assessed the majority of houses?
What are the results of your national assessment of the nation's housing stock ? Please share the results as I'm sure they would be of huge interest to the sector and industry.
Each individual situation needs to be looked at in terms of geographic location, property type (terrace, semi, detached, number of existing stories), construction type, incoming service locations, access for logistics and buildability, physical restrictions, boundary walls, trees with TPOs or which will impact any works, restrictive covenants or conditions by the area or local / national planning departments. Is the site level, on a slope? What are the ground conditions like? What depth would foundations be? Would you need mini piles ?
You simply cannot make that judgement call without that level of consideration
As for the use of space, again, it is dependant on so many factors - what if you have a 20ft wide hallway a staircase can go into and actually enhance the space rather than reduce it or make it less usable?
The roof construction may be of such that the height of the eaves / pitch of the roof gives you more than adequate space and headroom. There's then dormers and other solutions to give you additional space.
Again, in your second post, how many have you seen in each category you define ? One ? Five? Ten? It must be millions as you have claimed it is a "majority", you really do spout some utter ******** .