A rooflight is a hole in a roof, and a roof is a working structure. The visit into the loft tells us what the opening can be and where it can go. We look at the rafters, the insulation, any wiring or pipework crossing the line, and the condition of the covering outside.
If the rafters are sound and the spacing suits a standard frame, the opening can be cut and trimmed in the usual way, with a doubled rafter on each side and a header and sill across. If the spacing is awkward, we can often choose a unit width to fit it rather than cutting more timber than the job needs. Where a covering is brittle around the position, we flag it on the visit and the quote reflects it. It is no help to anyone to find that out when the first tile lifts.
We also look at which direction the slope faces. A roof window facing the sun gives a bright landing and, in summer, may need a blind. One facing away gives gentler, more even light. Neither is wrong. The decision depends on what the room is for.
Finally, we note how the loft is reached and where materials would be carried in. A narrow hatch or crowded landing is no obstacle, though it shapes the plan for the day, so we sort it out on the visit.