How to price an extension or dormer (carpenter)

Extensions and dormers are among the jobs where a carpentry quote most often slips. Not because the hourly rate is wrong, but because the construction gets priced as one lump. You remember the rafters but forget the wall plate. You put the tiles on but miss the connection to the facade. Suddenly the material list is 15% short. The fix is to price the extension for what it is: five separate layers, each with its own quantity and its own labour time.

An extension should be priced layer by layer, bottom up. Price each layer separately and you catch the items that otherwise fall between the lines:

• Foundation and plinth: excavation, reinforcement, casting, plinth render or blocks. Often a subcontract, but it must be in the total.
• Shell: load-bearing structure - rafters, wall plates, battens, posts, wind bracing. This is where the structural responsibility sits.
• Roof: underlay, counter battens, tiles or roofing sheets, ridge, flashings, roof pitch.
• Enclosure (building envelope): insulation, vapour barrier, wind barrier, external cladding, windows and doors.
• Interior: floor build-up, plasterboard, trims, architraves, interior finishes.

Once the split is in place, the customer can see what they pay for and you can see where your margin sits. This is also where quantity conversion becomes concrete: roof and facade in m², battens and wall plates in linear metres (lm), insulation in m² per thickness.

The shell is the backbone. The rafters carry the roof, the wall plate distributes the load to the wall, and the battens carry the roof covering. Quantities must be exact, because a mistake here costs both timber and hours:

• Rafters: count = roof length divided by rafter spacing (typically 900-1000 mm centres) plus one. Dimension follows span and load.
• Wall plates: linear metres along all load-bearing walls, plus joints and overlaps.
• Battens: roof area in m² converted to lm from the batten gauge, which follows the tile's cover height (e.g. 340-370 mm for concrete tiles).

When joining structural timber close to the end, pre-drill where there is a risk of the wood splitting. This is a labour item, not a material item, but it takes time and must be in the hours estimate.

The roof is where the water has to go, and where most callbacks come from. Count the underlay as roof area in m² plus overlaps and upstands. Choose the covering by pitch: tiles typically need at least 25 degrees, while low pitches need roofing sheets or a sealed underlay acting as an independent rain layer. Pitch and drainage are not a detail, they are the precondition for the roof to work.

Remember flashings and ridge as separate items. A dormer has more connections per m² than a plain roof surface, so labour time per m² is higher - that should show in the hours, not be hidden in a round m² price.

This is where the most-forgotten items sit, and they are legal requirements at the same time. The envelope must be airtight, and the connection to the existing facade must be watertight and moisture-proof. It is not optional:

• Insulation: counted in m² per construction part (roof, wall, floor) and by thickness per the energy requirement.
• Vapour barrier: fitted on the warm side, taped at every joint and sealed tight around penetrations. Tape and collars are their own material items.
• Wind barrier and external cladding: wind barrier outside, then counter batten and cladding (e.g. board cladding in lm or sheets in m²).
• Watertight connection to the facade: flashing, sealing tape and sealing where the extension meets the existing house. This is a requirement, not an extra.

Underestimate the sealing work and you get either a moisture claim back on the guarantee or an unhappy customer. Both cost more than the minute it takes to price the item correctly from the start.

The floor build-up depends on what goes on top. The standard build-up is battens on a moisture barrier, then chipboard or a load-distributing board. If there is underfloor heating, the board is grooved or the heat is laid in a screed - a different material and time item than a dry timber floor. Windows and doors are priced per unit with architrave, sealing tape and interior finishing.

The labour hours tie it all together. Count them per layer: shell raising, roofing, enclosure, interior. A dormer takes more hours per m² than a flat-roof extension, because there are more connections, more pre-drilling and more work at height. Never hide the hours in the material price - you lose the overview when the customer negotiates.

All the conversions - m² roof to lm battens, insulation area per thickness, rafter count from centre spacing - are where a manual quote either eats an evening or turns out imprecise. Calco works out the quantities from your measurements, splits the quote into the five construction layers, and includes the items that are easy to forget: vapour barrier tape, pre-drilling, watertight facade connection. Calco knows the moisture and airtightness requirements and does not let them drop out of the quote.

The result is a quote that is complete the first time, with a margin you can see, and evenings you get back. Try an extension: build your carpentry quote at /tilbud-tomrer.