This is the floor below, just to show how much of the front bedroom will be used to create the new staircase. The bathroom is in the top right corner of the photo

This is the party wall

The rear of the loft

Front of loft

Your stairs are in the best part of the loft. Already have some drawings from the architect? The drawings you did have too little info to be of help. Could use more measurements, existing walls, loadbearing ones. Will you add dormers?

Roof insulation too, I assume? Is the height with or without insulation? Roofs like these need to 'breathe', so please don't stuff the rockwool straight against the old wood and rooftiles.

Thank you for your reply, I have the architect on hold at the moment to rethink the design, the structural engineer said 2 supporting beams need to be installed but I’m unsure where. The roof will be retiled and a breathable membrane installed and there will be a full width dormer to the rear. Fire insulation will be added to the floor raising it by 50mm meaning the ridge height will be reduced to 2.4m

Would love to help, but not enough info. Are these the only drawings you have so far?

Yes, just rough drawings. I’m not getting the architect plans until we have decided the layout. What other info would you need?

Every current wall, windows, measurements, current bathroom layout. :)

The stairs to the cellar run directly under the main staircase

Am I correct the stairs on the left go up to the attic?

If it starts there, does it really take up 2/3 of the attic's width? How much distance between landing an exterior wall?

There are no stairs up to the attic, that is where we propose to put the stairs, taking up approx 1m of the front bedroom to create a corridor with winder stairs up to the attic

The area in red is currently the bedroom and the dashed line is where the corridor will be. The stairs next to the red area area the existing stairs from ground floor to 1st floor

Are they also taking up a meter of the attic or a little more?

They will be like this, so will protrude from the fable end wall less than a metre and be 2637mm wide

Did you have a structural engineer calculate the weight of a filled bath up there?

Like Jonathan already stated, this isn't going to be the spacious luxurious master suite you have in mind. If the majority of the stairs is on the floor below, you could have this layout, for example. The pale grey is the area where you can't stand up straight. Again, really really make sure the joistst can handle all this weight! Otherwise just turn this into a kid's loft and make a nice spacious family bath on the bedroom floor. With a separate toilet of course. ;)