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.

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.