Across 137 permits filed since 2025-05-04, the median is $40,000; half of projects fall between $20,000 and $115,000. Figures are permit valuations reported to building departments. National cost guides answer this question with averages from other markets; these numbers are what homeowners in Nassau County, NY actually reported when they pulled permits.
Recent filings
- INSTALL 2 PC BATHROOM AND WASHING MACHINE IN FINISHED BASEMENTRockville Centre · 2026-11-03$3,000
- FINISHED BASEMENT WITH 2 PC BATHRockville Centre · 2026-11-03$32,000
- The construction of a 34.5 x 32.83 (oa) (1,133 sf) second floor addition, interior alterations to existing first floor, an 18 x 15 (oa) (270 sf) one (1) story rear addition with basement, one (1) 6 ft wide sliding glass door, one (1) 9 ft wVillage of Mineola · 2026-10-26$383,811
How to read these numbers
Permit valuations are the project cost the applicant reports to the building department. They skew slightly low — nobody rounds up for the permit fee — and scope varies, so the middle-half range is the honest picture: half of all basement finishing permits in Nassau County, NY came in between $20,000 and $115,000. We exclude permits under $1,000, jobs filed as whole-house construction, and counties whose building departments don't reliably report valuations.
Same project, other places
Basement finishing in New YorkBasement finishing in ConnecticutBasement finishing on Long Island