Short answer: About 30% of the Town of Islip's land is inside an active sewer district on the county map, mostly Southwest Sewer District; about 44,741 of 70,341 one-family lots in the town's hamlets and villages sit outside any active sewer district. There is no town septic rebate, so the county and state grants are the funding.
Last updated: November 2026. Data computed 2026-11-07 from Suffolk County GIS; rules and programs re-read November 7, 2026.
Town of Islip in numbers
| Land in active sewer districts | 30% |
|---|---|
| Land in proposed districts | 16% |
| Under feasibility study (not a district) | 3% |
| Places with ~0% active sewer | 13 of 24 hamlets and villages |
| One-family lots outside active districts | about 44,741 of 70,341 |
| Groundwater zones (land share) | Zone VII about 43%; Zone I about 41%; Zone VI about 15% |
| I/A systems installed (Oct 2024) | about 240, 4% of the county's 5,344; 6th of the ten towns |
| Town septic rebate | No |
| Town code on I/A systems | None found beyond the county rules |
Sewer districts and where the cesspools are
- Active districts (4): Southwest Sewer District (about 28%), Parkland Sewer District (about 2%), Wind Watch Sewer District (under 1%), Birchwood Sewer District (under 1%).
- Proposed districts (13 polygons): MacArthur Airport/Industrial (about 5%), Oakdale Phase 5 (about 2%), Oakdale Phase 3 (about 2%), plus 10 smaller. Homes inside a proposed district are excluded from the county and state septic grants.
- Feasibility-study areas (not districts): Heartland Sewer Feasibility Study Area (about 3%). A study area does not by itself affect grant eligibility.
By one-family lot count, the most lots outside an active district are in Brentwood (about 9,147), Central Islip (about 5,418), Ronkonkoma (about 4,780), Holbrook (about 4,761), Sayville (about 4,388). Median one-family lots run from 0.10 acres in Ocean Beach to 0.49 acres in Great River. In 21 of 23 places the median lot is smaller than Article 6's minimum for a new home on an on-site system in its main zone. Across the town, about 9% of the land is in FEMA's 1%-annual-chance flood zone; 3% is in a Special Groundwater Protection Area.
What the Town of Islip adds
- Rebate: none listed by the county as of November 7, 2026.
- Town code: no town-wide I/A mandate found beyond the county rules as of November 7, 2026.
- Building department: Town of Islip; villages with their own: Brightwaters, Islandia, Ocean Beach, Saltaire.
Every hamlet and village in the Town of Islip
Land-area shares inside county sewer-district polygons; zone = main Article 6 groundwater zone; flood = FEMA 1%-annual-chance zone; lot = median one-family lot (acres). Linked names have their own page. Some village-run systems are missing from the county layer. Full table: sewer coverage by hamlet.
| Hamlet / village | Active sewer | Proposed sewer | Feasibility study | Zone | Flood | Median lot (ac) |
|---|---|---|---|---|---|---|
| Bay Shore | 96% | 0% | 0% | Zone VII | 7% | 0.22 |
| Bayport | 0% | 4% | 0% | Zone VI | 12% | 0.37 |
| Baywood | 0% | 1% | 0% | Zone VII | 0% | 0.37 |
| Bohemia | 0% | 16% | 0% | Zone I | 4% | 0.46 |
| Brentwood | 0% | 2% | 24% | Zone I | 0% | 0.25 |
| Brightwaters (village) | 100% | 0% | 0% | Zone VII | 8% | 0.32 |
| Captree | 0% | 0% | 0% | Zone IV | 78% | n/a |
| Central Islip | 23% | 2% | 0% | Zone I | 2% | 0.23 |
| East Islip | 92% | 0% | 0% | Zone VII | 9% | 0.26 |
| Great River | 57% | under 1% | 0% | Zone VII | 37% | 0.49 |
| Holbrook | 23% | 4% | 0% | Zone I | 0% | 0.24 |
| Islandia (village) | 8% | 0% | 0% | Zone I | 2% | 0.27 |
| Islip | 92% | 0% | 0% | Zone VII | 9% | 0.26 |
| Islip Terrace | 69% | 0% | 0% | Zone VII | 0% | 0.25 |
| North Bay Shore | 0% | under 1% | 0% | Zone VII | 0% | 0.27 |
| North Great River | 1% | 0% | 0% | Zone I | 3% | 0.46 |
| Oakdale | 0% | 98% | 0% | Zone VI | 35% | 0.32 |
| Ocean Beach (village) | 0% | 0% | 0% | Zone IV | 99% | 0.10 |
| Ronkonkoma | 1% | 36% | 0% | Zone I | 1% | 0.27 |
| Saltaire (village) | 0% | 0% | 0% | Zone IV | 100% | 0.19 |
| Sayville | 0% | 99% | 0% | Zone VI | 5% | 0.28 |
| West Bay Shore | 100% | 0% | 0% | Zone VII | 24% | 0.27 |
| West Islip | 91% | 5% | 0% | Zone VII | 7% | 0.26 |
| West Sayville | 0% | 100% | 0% | Zone VI | 28% | 0.26 |
County rules apply on top: no cesspool-for-cesspool replacement since July 1, 2019, and an I/A system for new homes and major reconstruction since July 1, 2021. Details, licensing and grants: Suffolk County overview.
Frequently asked questions
How much of the Town of Islip is on sewer?
About 30% of the land in the town's hamlets and villages is inside an active county sewer district, and 16% inside proposed districts (another 3% is under feasibility study, which is not a district); about 44,741 of 70,341 one-family lots in the town's hamlets and villages sit outside any active sewer district. A district boundary is not a connection.
Does the Town of Islip have a septic rebate?
No town rebate is listed by Suffolk County as of November 7, 2026. Islip homeowners use the county grant (up to $20,000) and the state grant (up to $25,000); the town had about 240 I/A systems by October 2024.
Who issues building permits for septic work in Islip?
The Town of Islip building department, except in the incorporated villages (Brightwaters, Islandia, Ocean Beach, Saltaire), which run their own. SCDHS approves the sanitary system itself.
Sources
Each source was re-read on November 7, 2026. Program amounts and rules change; the agency page is the authority.
- Suffolk County GIS Viewer map service (hamlets, villages, sewer districts, groundwater zones, flood zones, tax parcels)
- Suffolk County DHS presentation, 11/19/2024 (hosted by the Town of Shelter Island)
- Suffolk County DHS: Changes to wastewater practices effective July 1, 2019
- Suffolk County: Septic Improvement Program information
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