We publish no prices, and we can be specific about what moves the number. On a flat roof the unit is rarely the largest line. The condition of the kerb, the covering termination, the access and what is found once the old unit comes off generally account for more than the rooflight itself.
The unit
The part people expect to dominate, and it varies less than the rest.
Size, whether it opens, whether it is powered, and the glass specification are the variables, and they are all known before anything starts.
A standard size costs less than made to size, which matters on a replacement into an old opening.
The kerb, which is the real variable
The line that separates a morning from a day.
A sound kerb at the right height, square and insulated, means the unit goes straight on. One that is too low, racked or uninsulated means building work before the rooflight is touched.
What is usually wrong is on what is a builders kerb.
Raising a low kerb
Ordinary carpentry and covering work, priced separately so the choice is visible.
It means building up in timber, extending the covering up the new face and terminating it properly, which brings a roofer into the job.
It is not optional where the existing height is inadequate, because the height is the weatherproofing.
Insulating a kerb
A smaller line and it prevents the commonest callback.
An uninsulated upstand produces condensation running down the frame every winter, which gets reported as a leak. Insulating it during the work is a fraction of returning to do it.
The covering
Which covering it is decides who does the termination and what it costs.
Felt, single ply, GRP, liquid and asphalt each terminate differently, and asphalt in particular is specialist work most domestic installers do not do.
What each involves is on can you fit a rooflight on a felt roof.
The condition of the covering
The item that occasionally changes the whole recommendation.
Where a covering has a couple of years left, we will say that renewing it and forming the opening as part of that work costs less overall than doing them separately.
That advice loses us a job now and it is obviously right.
Getting to the roof in How much does a flat roof rooflight cost?
Generally the easiest part on a single storey rear extension.
A tower on a drive with a side gate is a modest cost. A terraced house with no side access, where everything goes through the building, is not.
It is established at survey and quoted as its own line rather than absorbed.
Getting the unit to the roof
A cost that appears on larger units and surprises people.
There is a size beyond which a made to size unit will not come through a house or round a side passage, and beyond that a hoist or a crane is involved.
It is measured with a tape, and it is frequently why two smaller units are proposed.
Structural work
Modest on an ordinary opening, real on a large one.
Trimming between joists is carpentry. A wide opening needing a substantial trimmer or a beam is a designed detail with a structural cost attached.
Making good inside
Frequently underestimated on a retrofit. Plasterboard patches easily. A reveal formed through a thick warm roof build up, splayed and finished, is a plastering job of its own.
Splaying it is worth the money, because it delivers noticeably more light.
What we find on opening up
The line that cannot be priced in advance, and we say so.
Soft timber at the base of an upstand, a covering perished at the termination, insulation that turns out not to be there.
We price the known work and state clearly what would be extra, rather than quoting a firm number that does not survive the first hour.
Electrics
A small line where a powered unit or blind is wanted.
A supply at the opening is minutes of work while a ceiling is open and a separate job afterwards, which is why we recommend running a cable even where a manual unit is chosen.
Two units against one
Where the arithmetic goes the way people do not expect.
Two modest units frequently cost less to manufacture than one of the same combined area, light a long room better, and are easier to get to the back of a house.
The reasoning is on how many rooflights does a flat roof extension need.
Why two quotes differ
What is included rather than what the total says.
A quote assuming a sound kerb on a roof that needs one raised is cheaper for a different job. One excluding access, or the covering work, is not comparable.
We itemise so the comparison can actually be made.
Where the money is well spent
The parts nobody sees afterwards. The kerb height, its insulation, and the covering termination decide whether the installation is sound in twenty years, and they are exactly what a cheap quote leaves out.
Where it is not
Two places, and we will say so.
A larger unit than the room needs, and solar control glass on a north facing deck. Both add cost for no benefit.
How a price is arrived at
A free indicative range by email from photographs, then a survey, then a fixed itemised written price that does not move afterwards.
The method is on the how we price and quote page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.