Two homes with the same room can receive very different quotes, and the reasons are mostly down to the roof and the unit. Because the fixed figure is written after the survey, it reflects your house. The table below shows the main things that move it, in plain terms and without any numbers, since the numbers belong in your own quote.
Opening width heads the list, because a bigger unit means longer trimmers, a wider light well and more plasterboard. The second is the depth of the void between roof and ceiling. A shallow void gives a short well, a tall loft gives a long one. The third is the roof covering, since slate, tile and other coverings each need their own flashing.
Access is the next. A house with a clear approach and a hatch beneath the opening is quicker to work on than one where scaffolding or a long carry is needed. Fourth, the choice of unit. A fixed pane, a manual opening one and an electric one are all available, and they differ in cost.
Weather and exposure count too. A roof that takes the wind needs well-secured flashing, and a slope that holds snow for longer needs a unit with a suitable seal and frame. We note the exposure of your roof at the survey and allow for it in the specification.
Finally, finish. If the well is to be plastered and painted, that is part of the job, but if you are decorating the room anyway, the quote can reflect that. We write what is included on the quote, so nothing is left to assumption.