hi,
as architects we notify clients that this can be a good option to make the loft feel like a normal height space, especially when the existing ridge is not as high as we would like.
You can raise the ridge approx 250mm so long as you do it in the same plane as the roof front (subject to planning and if in conservation area etc) - and make sure your builder avoids putting in an awful step up here which a lot of bodgers do.
Also: only do this if you still have enough height over your windows to install curtain rails. you can locally create step ups around windows. we have done this previously, as you install a steel approx 1m from the front wall.
I would suggest you clear all your first floor rooms as the dust will get everywhere.
demolishing the first floor ceiling turns a standard loft conversion into a more major build. I would suggest you move out - which would add a few £ks.
Builder would need to charge for more waste removal, and more plasterboard and to refinish rooms at first floor etc. Builder should be able to identify a clear fixed cost for you.
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