Enter your measurements to get roof area, a materials list and an estimated construction cost. Results update as you type.
Generated at dormercalculator.com on October 26, 2026
Width: 10 ft | Depth: 8 ft | Wall height: 4 ft | Roof pitch: 4 : 12
Finish: Fiber cement + architectural shingles
$16,604
Likely range $14,110 to $19,090
About $208 per sq ft of new floor space.
| Item | Quantity |
|---|---|
| Wall studs2x4 or 2x6 at 16 in centres, incl. corners | 24 pcs |
| Roof rafters2x8 at 16 in centres, about 8.9 ft long | 9 pcs |
| Structural header beamLVL or steel, spans the roof opening | 11 ft |
| Sheathing4 x 8 ft sheets, walls and roof, 10% waste | 8 sheets |
| Roof coveringarchitectural shingles, 10% waste | 98 sq ft |
| Wall claddingFiber cement, 10% waste | 138 sq ft |
| InsulationWalls and roof surface | 215 sq ft |
| Plasterboard / drywall4 x 8 ft sheets, interior walls and ceiling | 8 sheets |
| WindowsRoughly one unit per 5 ft of width | 2 pcs |
| Work item | Cost |
|---|---|
| Roof opening, header beam and structural work | $2,000 |
| Floor framing and subfloor | $960 |
| Wall and roof framing with sheathing | $4,723 |
| Roof covering and flashing | $688 |
| Wall cladding | $1,517 |
| Windows (installed) | $1,500 |
| Insulation | $644 |
| Drywall and interior finishing | $966 |
| Electrical and heating extension | $1,440 |
| Contingency (10%) | $1,444 |
| Design and permits (5%) | $722 |
| Estimated total | $16,604 |
Planning estimate using average US installed prices. Not a quote. See the disclaimer for details.
The calculator converts your four measurements into surface areas, then multiplies those areas by material and labor rates. Here are the formulas it uses, so you can check any result by hand.
Roof rise
rise = depth x pitch / 12
Rafter (slope) length
slope = square root of (depth squared + rise squared)
Roof area
roof = width x (slope + 0.5 ft overhang)
Wall area
walls = width x height + 2 x depth x (height + rise / 2)
Floor area
floor = width x depth
Order quantities
quantity = area x 1.10 waste allowance
Total cost
total = (labor + materials) x 1.10 contingency x 1.05 design and permits
Pitch is entered as rise per 12 inches of run, for example 4 means 4:12 (about 18.4 degrees). Lengths are converted to feet internally, so metric input gives the same answer.
Real-world sizes calculated with the same engine as the tool above. Use them as a reference for your own project.
Example 1
A bright reading nook in a 1930s semi-detached attic. Mostly about light and curb appeal.
Estimated total
$8,779
Range $7,460 to $10,100
Example 2
A rear shed dormer that turns a cramped attic into a proper bedroom with headroom.
Estimated total
$20,963
Range $17,820 to $24,110
Example 3
A wide premium shed dormer with metal roofing, adding a bedroom and ensuite bathroom.
Estimated total
$39,917
Range $33,930 to $45,900
The calculator gives a planning-grade estimate, typically within 15 percent of real-world quotes for standard builds. It uses average US material and labor rates. Local prices, site access, structural surprises and design complexity can move the final cost, so always get written quotes.
Most dormers cost between $15,000 and $80,000 installed. Small decorative dormers cost less, and wide shed dormers that add a full room cost more. Use the calculator for a figure based on your exact dimensions.
It depends on where you live. Many rear dormers are allowed under permitted development in England, while most US towns require a building permit. Conservation areas, listed buildings and flats have stricter rules. Always confirm with your local authority.
A shed dormer has a single sloping roof and gives the most interior space for the cost. A gable dormer has a pitched roof with a triangular front, looks more traditional, and is more complex to build.
Shed dormers typically use between 3:12 and 6:12 so water drains well and headroom stays generous. Gable dormers usually match the main roof pitch. Anything below 2:12 needs specialist roofing.
Yes. The cost estimate uses installed unit prices, which include both materials and labor. The materials list shows the quantities you or your contractor would order.