Across 135 permits filed since 2025-05-01, the median is $40,000; half of projects fall between $20,000 and $120,500. Figures are permit valuations reported to building departments. National cost guides answer this question with averages from other markets; these numbers are what homeowners on Long Island actually reported when they pulled permits.
Recent filings
- 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
- Finish basement laundry roomVillage of Garden City · 2026-10-20$56,840
- CONSTRUCT ONE AND TWO STORY REAR ADDITIONS (803 SQ. FT.), REAR COVERED PATIO, FINISHED BASEMENT, INTERIOR ALTERATIONS BUILT IN ACCORDANCE WITH NYS AND RVC CODE. DRYWELL, VFR TRUSS PLACARD AND FINAL SURVEY REQUIRED FOR C/O. SEPARATE PERMITRockville Centre · 2026-10-16$290,000
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 on Long Island came in between $20,000 and $120,500. 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 Connecticut