In the construction menu there are already finished buildings, do those big constructions only count as 1 in the settlement size for example in comparison with 1 junk fence object?

Settlement size or the 'budget' yoi see meter on is a collection of various factors, without going inyo graphics geek talk, just use rule of thumb that the 'bigger' and 'more detailed' the object, the more polys and such go up for the budget count.

So if you create one of the big prefab buildings, no - it doesnt count as just 1 object like any other object. it uses more than a smaller, less detailed construction.

I have built wooden platforms on top of three of the existing houses using the base with stilts (so it perches the same height as the apex of the existing house roof. I have then built up new settlements on these platforms. How does this effect it's ratings as technically I am adding to an existing dwelling?

Every object in the settlement, including existing ones, has a polygon count in terms of triangles and the way FO calculates the budget per object. So whether you use the platform stilts or other objects, you increase the polygon count by building.

If you want to build essentially your own settlement on top of the existing one (which I do as well) - the best thing to do is scrap every single thing that you can of the existing objects. Some places like sanctuary have a lot of pre-existing stuff that can't be scrapped, but some plots you can get rid of a good part.

I don't know offhand whether the raised platform stilts are lower or higher in poly count than other risers, but I would expect maybe bit less given it has less detail and fill space than say concrete foundations which fill the entire point between any X, Y, Z coordinates.

Common sense rule of thumb works fairly well - the more you think a graphics engine has to 'draw' an object, the more budget it will take from your settlement.

If you are on PC, all this is moot point as you can get mod that raises your base budget by ~3x, which is more than enough for most people. Above that you can go custom but you'll need a pretty high end rig to handle whatever fuels your imagination as far as super building.

I had an issue along these lines yesterday and someone told me this and it worked : any guns you collect take them home and drop them , enter build mode and scrap them on the floor , it tricks the game into thinkin your reducing the size and the size meter comes down , I tried it with all random junk and it didn't work , just guns , I'm on ps4