What Does a Dormer Addition Cost on Long Island in 2026?

Every dormer quote we write in 2026 falls between $95,000 and $285,000. That is a wide band because a dormer is not one product — it is a category that covers everything from a 14-foot attic egress dormer to a full-width Cape conversion that effectively doubles a house. Below are the ranges we see most often, what assumptions they ride on, and how to read a bid so you know what you are paying for.

2026 cost ranges by dormer type

These are turn-key ranges — design, permits, demo, framing, roofing, siding, insulation, electrical, plumbing if applicable, drywall, trim, flooring, paint, and clean-up. They assume a house in reasonable structural condition and a scope delivered on a Long Island address in Nassau or Suffolk County.

Attic / egress dormer — $45,000 to $85,000

Smallest job we take on a dormer-only scope. Adds code-compliant egress, headroom, and daylight to an existing finished or semi-finished attic. No plumbing. Assumes the existing roof is under 15 years old and does not need full replacement.

Partial shed dormer — $95,000 to $145,000

Typical scope: one new bedroom plus a hall bath, roughly 350–550 sq ft of usable space. Assumes one-day roof strip, modern framing lumber, standard-grade finishes, and no HVAC system replacement (extends existing forced-air or adds a mini-split).

Full shed dormer — $145,000 to $215,000

Our most common dormer job. 800–1,200 sq ft of new second-floor space, typically a primary suite plus two additional bedrooms or a bedroom plus home office. Assumes mid-grade finishes, all-new electrical on the new floor, and full tie-in to existing systems.

Full Cape conversion (front + back) — $195,000 to $285,000

Effectively a new full second story on a Cape Cod footprint. Includes a new roof over the entire house, full electrical upgrade to 200-amp service in most cases, and structural reinforcement of existing bearing walls and footings.

What moves the number inside a range

Five factors push a quote toward the top or bottom of the range above:

• Structural reinforcement. Pre-1960 framing often needs sistered joists, new LVL beams, or footing work. Add $8,000–$25,000.
• Roof replacement scope. If the existing roof is end-of-life we replace the whole thing, not just the affected section. Add $18,000–$32,000.
• Finish level. Custom millwork, tile selection, and cabinetry can swing the number 15–30% on any job.
• Electrical service upgrade. Most 100-amp services need a 200-amp upgrade to support a second floor. Add $4,500–$7,500.
• Town-level permit posture. Hempstead and Islip are efficient. Southampton, East Hampton, and North Hempstead take longer and sometimes require variance fees. Carrying cost impact: $3,000–$12,000.

Assumptions we are NOT making

The numbers above do not include:

• Asbestos or lead abatement on pre-1978 homes.
• Septic or cesspool expansion (required in some Suffolk towns when adding bedrooms).
• Complete HVAC replacement for whole-house systems over 15 years old.
• Exterior siding replacement on the existing house to match the new dormer.
• Interior renovation of the existing first floor.

When any of these show up during the walk-through we flag them immediately and give you a separate line item so nothing is buried.

Timeline you should expect in 2026

Our current lead times on Long Island:

• Walk-through to fixed-price quote: 10 business days.
• Quote signed to permit in hand: 6–14 weeks, town dependent.
• Permit in hand to framing start: 2–4 weeks.
• Framing start to final inspection: 10–16 weeks on a full shed dormer.

Total project clock from your first call to the final walk-through averages 6–9 months on a full dormer.

Village-specific pricing: Garden City

Garden City dormers carry a 10–20% premium above standard Nassau County pricing due to the Village Building Department permit process and, for Historic District homes, the Architectural Review Board review. The ARB requires material samples and elevation drawings showing the dormer in architectural context with the existing home — matched brick coursing, sand-float stucco, period-appropriate windows. A shed dormer on a Garden City colonial runs $110,000–$145,000 all-in; gable dormers on Tudor revivals run $90,000–$125,000. See our full Garden City dormers cost and process guide for a detailed breakdown.

How we give you a real number

We do not quote dormers off a phone call. Every number we put on paper follows a measured walk-through, sketch review, and preliminary structural assessment by our in-house PE. The fixed-price quote you sign is the number you pay, with itemized allowances for the finish-level decisions that are legitimately yours to make.