Dormer Installation Cost Calculator

What this calculator estimates

This calculator quotes the all-in installed price for a roof dormer addition in 2026 US dollars. It covers the framing, exterior sheathing and roofing, weather-tight envelope, optional window, optional drywall and electrical interior finish, building permit, and an optional crane day for large or eyebrow dormers.

The bill is split into the line items real dormer contractors invoice:

• Framing and roofing — the bulk of the job. Cut the existing roof, install headers and rafters, sheathe and paper, install matching roofing, side the dormer cheeks, and flash the intersection with the main roof.
• Window unit — one window per dormer by default. Larger dormers may take two — adjust by toggling the window line off and pricing the window separately if you need an oversize unit.
• Interior finish — drywall, trim, baseboards, paint, electrical rough-in (one circuit, two outlets, one switch, one fixture), and final inspection.
• Building permit — local AHJ (authority having jurisdiction) fee. Adds 30 to 60 percent on top of base permit in historic districts.
• Crane day — required for large or eyebrow dormers where pre-built wall and roof sections are lifted to the roof in panels. Optional on small and medium dormers where stick-framing is faster than crane mobilisation.

A minimum job fee of $3,200 applies in most US metro markets. Small dormers under 3 ft x 4 ft hit the floor because mobilising a framing crew, scaffolding, and a roofing crew is the dominant cost.

How to use it

1. Number of dormers — enter how many dormers you are adding in one continuous build. Two dormers in the same scope share permit and crane mobilisation costs and save 10 to 15 percent each compared to two separate jobs.
2. Style — shed for budget, gable for curb appeal, hipped for premium look, eyebrow for custom historic restoration.
3. Size — small (3x4 ft, attic light), medium (5x6 ft, bedroom window), large (8x10 ft, primary bedroom expansion).
4. Construction type — new build (framed during initial roof construction) is 15 percent cheaper because there is no tear-out, weather risk, or matching-shingle search.
5. Building height — single-storey is the baseline. Two-storey adds 10 percent for ladder repositioning and scaffold. Three-storey adds 25 percent for swing-stage or boom-lift.
6. Site access — easy (clear ground, dumpster access), moderate (some shrubs, normal setback), difficult (zero-lot setback, power lines, lift required).
7. Window unit — defaults ON because almost every dormer has a window. Toggle OFF if you are adding a decorative dormer with no opening.
8. Interior finish — toggles drywall, trim, electrical rough-in, and final inspection. Toggle OFF if you are leaving the attic unfinished or finishing it as a separate later contract.
9. Building permit — defaults ON. Toggle OFF only if you have already paid the permit fee outside this scope.
10. Crane day — toggle ON for large or eyebrow dormers, or for two-or-more-dormer jobs where pre-fabricated wall and roof panels save 2 to 4 days of stick-framing on site.

Typical 2026 US dormer cost ranges

Add 8 to 15 percent in coastal salt-spray regions for marine-grade fasteners and aluminum or copper flashing.

Cost drivers

Style. Shed dormers are 20 to 30 percent cheaper than gable for the same footprint because they only need one roof plane and no end walls. Gable adds two pitched roof planes, vertical end walls, and a more complex valley intersection. Hipped adds four roof planes and far more flashing perimeter. Eyebrow is the most expensive because the curved roof framing must be custom-bent or built up from radius rafters and laminated sheathing — figure 60 to 110 percent more than gable.

Size. The labor curve is roughly area to the 0.7 power, not linear with area — a dormer twice the size does not cost twice as much because mobilisation, permit, weather-tight envelope, and crane day are roughly fixed. Doubling the linear footprint typically adds 60 to 80 percent to the framing line.

Retrofit vs new-build. Retrofit adds 12 to 18 percent because the existing roof must be cut, weather-tight tarping must be staged, salvage-matching shingles must be sourced (or the entire roof slope must be re-shingled to avoid a colour mismatch), and existing electrical or HVAC ductwork may need to be re-routed. New-build during initial roof construction skips all of that.

Interior finish. Drywall, trim, and electrical add 25 to 40 percent to a basic dormer shell. Add another 10 to 20 percent for upgraded finishes (hardwood floor extension, built-in window seat, custom millwork).

Site access. Single-storey ranchers are easy. Two-storey colonials are moderate. Three-storey row-houses, beachfront properties with zero-lot setbacks, and houses under power lines all require lift rental and add 20 to 40 percent.

When to add a dormer versus when to expand differently

Add a dormer when:

• The attic has 7/12 (30 degrees) or steeper pitch and at least 7 ft of ridge-to-floor height.
• You need 60 to 200 sqft of additional living area for the lowest cost-per-sqft.
• The attic floor structure is sized for living load (or can be sistered to bring it up to code).
• The roof has 5+ years of remaining life and is in good condition (avoid adding a dormer to a roof you will tear off in 3 years).

Expand differently when:

• The attic pitch is under 5/12 — usable head-height is minimal and the dormer cost-per-usable-sqft is poor.
• The structure is engineered trusses — cutting trusses requires engineering and the cost approaches a small addition.
• You need 300+ sqft of new space — a ground-floor addition or a full second storey is usually cheaper per usable sqft.
• The existing roof is 15+ years old — re-roof first or bundle the dormer into a re-roof project.

What to look for in a contractor

A competent dormer contractor will:

1. Pull a building permit and provide the permit number before tear-out starts.
2. Provide a written weather-tight plan including tarp staging, daily forecast checks, and material-ready trigger criteria.
3. Step-flash the dormer-to-main-roof intersection per IRC R905 — woven into each shingle course, not surface-applied.
4. Restore attic ventilation per IRC R806 — soffit vents in the dormer cheeks, continuous ridge vent (or equivalent) along the new dormer ridge.
5. Match the existing roofing material, colour batch, and exposure — or quote a full-slope re-roof to avoid the inevitable colour mismatch.
6. Coordinate with the electrician if interior finish is included — outlets, switch, and fixture rough-in before drywall.
7. Schedule the building inspection before drywall and before final closeout.

Red flags: no permit, no written weather-tight plan, surface-applied flashing, refusal to discuss ventilation restoration, no structural engineering on a truss roof, sub-$200/sqft pricing on a finished-interior dormer.

Code references and standards (US)

• IRC R301 — Design criteria (snow load, wind load, seismic).
• IRC R310 — Emergency escape and rescue openings (egress window requirements for sleeping rooms).
• IRC R802 — Wood roof framing (rafter, header, and valley framing rules).
• IRC R806 — Roof ventilation (soffit-to-ridge ratio).
• IRC R905 — Roof covering and flashing (step-flashing at sidewalls and dormer cheeks).
• ASTM E2112 — Standard practice for installation of exterior windows, doors, and skylights.
• OSHA 29 CFR 1926.501 — Fall protection above 6 ft (mandatory on dormer framing).
• NFPA 70 (NEC) Article 210 — Branch circuit requirements (if electrical is added).

Diagnostic checklist before signing a contract

• Attic head-height measured at the proposed dormer location.
• Existing roof framing identified (stick or truss) — truss roofs require engineering.
• Existing roof material and colour batch identified for matching.
• Egress requirements confirmed if the dormer creates a sleeping room (IRC R310).
• Ventilation calc included in the contract (soffit-to-ridge ratio after the dormer).
• Weather-tight plan in writing.
• Permit fee included as a line item, not a hidden adder.
• Inspection schedule listed (rough framing, electrical, final).
• Warranty terms specified for framing, roofing, and window separately.

Related calculators

• Roof replacement cost calculator — bundle dormer with re-roof for material match.
• Skylight installation cost calculator — cheaper alternative when you only need light, not floor area.
• Roof flashing cost calculator — flashing line is critical at the dormer-to-main-roof intersection.

Sources: 2026 GAF and Owens Corning installed-quote data; NAHB Cost Versus Value Report 2026; IRC R301, R310, R802, R806, R905; ASTM E2112; OSHA 29 CFR 1926.501; NFPA 70 NEC; Boston, Chicago, Seattle, Atlanta, and Denver dormer-specialist contractor benchmarks.