Can you add a skylight to an existing roof?

Timber spacing comes first. Roof windows come in a range of widths, and where the window is wider than the gap, the rafter is cut and new timbers carry its load to the ones on either side. We measure the spacing, the size of the timbers and any signs of earlier alterations.

After that comes whatever else lives up there: a water tank, pipework, cables, ductwork from a bathroom fan, a chimney stack or a party wall close by. Any of these can move the position of the window by a few hundred millimetres, or call for a smaller one. Insulation lies between and over the timbers, and it is cut back and reinstated around the lining so heat does not leak round the edge.

If the space above the ceiling is deep, as it is above a landing under a pitched loft, a lined shaft carries light from the roof to the ceiling.

Peg tiles, plain tiles and natural slates are lifted around the opening and relaid onto a flashing made for that covering. Profiled tiles are dressed differently, and an unusual covering is discussed with you before we agree the work.

On a flat roof, the deck is cut and an insulated kerb is built, with the membrane dressed up its sides. A flat rooflight sits on that kerb, and a roof lantern sits on a larger one. A lantern's glazed body usually rises past the 150 mm permitted development limit, so it is normally a matter for planning permission, unless the council tells you otherwise. If a flat roof has a gentle fall, the pitch of the roof is part of what we assess; our note on the lowest pitch a roof window suits explains why.

Flat roofs take flat roof rooflights where the roof is low, or roof lanterns where the room below is wide.

A downstairs hall or a first-floor landing may have a bedroom or an attic above it, so the roof is not directly overhead. A sun tunnel can bring light across the gap through a tube from a small opening on the roof; ours are covered under sun tunnels. Where the loft is only storage, a roof window can serve the room below if the timber and ceiling above can be lined as a shaft.

We discuss this at the survey.

A window placed over the brightest corner of a room adds little. Over the part of the floor that sits furthest from the glazing, it adds the most. In a chalet bedroom with a dormer at one end, the bed wall at the other is usually the dark side. In a bungalow with a deep hall, the middle of the passage is. An en suite under the eaves may only have room for a small unit between two rafters, and that is fine if it sits over the basin.

Height also counts. A window set low in a steep slope gives a view out when you sit, and one set higher gives more sky when you stand. We mark the opening on the ceiling and the roof together, so the line of the timbers and the line of the room agree before anything is cut.

Planning comes first, and the rule that matters first is the projection limit: a rooflight or roof window is usually permitted development if it rises no more than 150 mm above the original roof slope, frame included, and sits no higher than the highest part of the original roof. Flats and maisonettes sit outside householder rights altogether. The side-slope condition, the listed-building position and the Article 4 question are set out in our planning permission guide, which also explains why living in a conservation area is not by itself the barrier it is often thought to be.

The second set of rules is building work. Any new opening in a roof involves the structure and the energy limits, and a new roof window must meet 1.4 W/m²K, or Window Energy Rating band B. Where the work is notifiable, we handle the Building Control notification.