Lowering loft floor for dormer height

Hi all, so I’m going to be having a loft conversion with a dormer at the back. now the New loft floor joists will need installing lower than the Old ceiling joist to make extra height for The loft conversion.. structural engineer has said to use 8 x 2 joists for the new floor and roof in the loft.

the issue I have is that when the new joists will be placed lower and once the ceiling is plaster boarded in the rooms below there won’t be much space from the top of the window openings to the New ceiling. What is the best course of action in this scenario? Would it be to place a concrete lintel lower than the top of the window opening then brick up the remaining gap above so I can have a nice uniform gap from the top of the window to the new ceiling level.? another small issue is that the front of the house has two bays ground floor and first floor with brick pillars obviously it’s only the first floor that’s gonna be affected by this situation but will I need sandstone stone lintels for the top on the outside of the bay.. nobody has a dormer on the street (yet)..

8x2 for floor is common, but it's usually possible to use more shallower timbers if headroom is critical.

8x2 for the roof seems excessive though: I can't recall ever going this big in >20years of loft design. One dodge I often used to gain an odd inch was to size the joist as normal but then cut from the centre to the end at 1 in 60 to give the fall and flip the off cut to complete fall end to end. Also the centres can be closed up as above. Finally a cold decj will give you more headroom than a warm one though the latter is much preferred.

I think your SE needs to have a think about all the constraints involved with your job and do a better job for you.

I wouldn't mess about with the windows if it was mine.

yes I think he’s used these sizes because there is going to be 150mm of insulation between the rafters.. i think I will Just have to leave a 150mm gap from the top of the window to the bottom of the joist hanger plate. That will allow me to have a decent coving and it won’t look silly. I’m hoping it won’t reduce the finished loft height too much. I will however speak to the engineer again.!