Q:

Curved roof (attic)?

Apologies if this comes across as a stupid question, but I was wondering what might be the best way to achieve the curved attic (roof?) part of this building? 

My gut says add a cube and add some cuts to pinch in the middle sections, but I'm not sure how to convert that into a smooth curve!
Any help would be greatly appreciated.

A:

As has been commented, a subdiv. approach might be:

To inset the top face of a cuboid evenly: I inset it with 'Even' checked..
Select the outside vertices and  GG slide the loop inwards
AltM merge the top vertices by distance.

CtrlR cut loop round the waist
Assign a Bevel modifier, by angle.
These are my Bevel settings, quite a common way to put in holding loops to contain  Catmull-Clark subdivision, in the right circumstances. 

(You can, instead, use Bevel Weights, or introduce holding loops by hand)
Assign a Subdivision surface modifier, probably 1 level of Catmull-Clark will do

You can adjust the curvature by adjusting the one loop around the waist, (at least, to block the model out).

