Dormer window query

[lewis&cherie]

Our house used to have a mezzanine level on the first floor before the previous owners added joists and extended the floor over. This increased the floor space into the eaves as the roof on one side of the house slopes down to first floor level.

What I would like to do is have a pitched roof dormer window installed into the roof so that there is headroom and the space becomes useable. The size of the dormer will need to be about 8 foot wide and go 12 foot deep at the pitch of the roof. I have checked with the council and planning permission is not needed just building regulations.

Before I take it any further, as a rough guide, does anyone know how long it would take to install a dormer window this size (Just concerned about the weather elements) and also any idea of the costs involved?

I would be very grateful for any answers�Thank you