First thing to do is check the sales plans. I doubt this is shown.
I suspect this is boxing in something, pipes probably or the wall hasn't been set out correctly from the other side and this jut out is needed to fit in an en-suite or airing cupboard.
I doubt he site manager has seen this yet or if he has, he isn't that bothered about quality.
To make it even worse for you the window is now not central between the walls. The radiator will also be offset from the centre of the window or the room. Worse, you have no room for curtains to hang down when open on the left hand side.
If this is not shown on the drawings them you must insist it is put right.
No you are not being awkward, this will be your home and no doubt is costing you hundreds of thousands of pounds. Tell them you will give them a one star rating if they don't give you a good quality new home.
AS I always say.. "The squeaky wheel gets the oil" so complain and be a nuisance.
Please name the housebuilder.
