Im buying a traditional terrace property. Currently the downstairs is open plan and the stairs start at the rear of the house rising up toward the from door. kinda backwards from what you might expect.

I want to move the stairs so the stairs split the downstairs back into 2 rooms. this way upstairs we can knock bed 2,3 and the current landing together, create a corridor to the rear and by extending over the current kitchen, a 7ft x 9ft family bath.

we think the joists run front to back along the house, and we would utilize the current "gap" at the top of the stairs so we think we just need to put back one joist to create the corridor. Anyone done this and have any idea as to costs?

In addition i want to extend the kitchen into the side return and knock the whole back section into a open plan kitchen/diner/living space with loads of glass leading onto the courtyard and garden. the extension would be about 16ft x 9ft the current bathroom would get knocked down as there would be no courtyard else.

we are considering what to use for the bathroom/ kitchen extension. Currently the kitchen has a flat roof. unsure about how good the foundations are so we were thinking of SIPS panels with flat roof for at least the bathroom? quick, easy, light? again anyone done this?
And again SIPS or bog standard lean to conservatory type structure for kitchen? it would only be one long side (full wall no glass) plus the end gable (full glass) and i want a glass roof so i think a conservatory would be the simplest and cheapest way to go.

anyone done this? thoughts? Im just looking for estimates of structural costs currently as we have about 20k for all this work so im guessing we will need to do the plastering, electrics plumbing etc as we go.

Thanks for your input :)