why didn't you consider the rooflight when you had all the people able to advise best involved? They should have specified the lantern to ensure all the structural requirements and insulation required were dealt with rather than leaving you to your own devices on a computer forum!
Adding it after all the drawings are done leaves you open to costs - the builder will have to reprice the roof with the additional trimming of joists and upstands required etc, more plastering, changes to electrics etc etc
As you need a party wall agreement presumably you are building on the boundary - have you considered neighbours upstairs windows if you go with a lantern type? i.e. can they see in from upstairs windows?