extending over staircase

[lauralai]

Hello,
I'm after a little advice as building is a subject I know very little about.

I have found my dream house on the market, the only problem being that even though it is listed as a 3 bed, the 3rd bedroom isn't actually even big enough to fit a bed in! Its more of an office. However, I would need it as a bedroom and was wondering if it is possible ti extend over a staircase. It is the opposite end to the opening of the stairs so it shouldn't affect using the stairs.
Unfortunatly this seems to be the only way to extend due to the big bay windows.

Any advice would be much appreciated,

Thanks,
Lauralai