Short answer: Most of West Islip is inside an active county sewer district: about 91% of its land lies within Southwest Sewer District.
West Islip in numbers
| Town | Town of Islip |
|---|---|
| Area | 6.12 sq mi |
| In active sewer district | 91% · Southwest Sewer District |
| In proposed district | 5% · Unfunded Carlls River 110-1 |
| Groundwater zone | Zone VII 100% |
| FEMA flood zone | 7% |
| One-family lots / median | 7,967 / 0.26 ac |
| One-family lots outside active sewer districts | about 564 |
Computed 2026-11-07 from Suffolk County GIS (hamlet boundaries, sewer districts, groundwater management zones, FEMA flood zones, protection areas, tax parcels). Shares are of land area; a district boundary is not a hookup, and some village-run systems are missing from the county layer. Method and full table.
Sewer coverage
Most of West Islip is inside an active county sewer district: about 91% of its land lies within Southwest Sewer District. That still leaves pockets of West Islip where homes were never connected and run cesspools or septic tanks. A sliver (5%) is inside a proposed district (“Unfunded Carlls River 110-1”). Counting lots instead of land: about 564 of the 7,967 one-family lots touching West Islip lie outside any active sewer district, and those homes rely on on-site systems. For comparison, the nearest places: West Bay Shore (100% sewered, median lot 0.27 ac); Brightwaters (100% sewered, median lot 0.32 ac); North Babylon (69% sewered, median lot 0.70 ac). By active-sewer share West Islip ranks 13 of the 165 Suffolk hamlets and villages we measured.
Groundwater, lots and flood exposure
Groundwater management Zone VII: Article 6's minimum lot for a new home on an on-site system here is 20,000 sq ft, unless the density is met another way or an exemption applies. County parcel records show 7,967 one-family lots touching West Islip, with a median of 0.26 acres (about 11,200 sq ft), below that minimum and the 124th-largest median lot in our Suffolk table. 7% of the land is in FEMA's 1%-annual-chance flood zone.
If your West Islip street isn't connected, the same county replacement and grant rules apply as anywhere in Suffolk.
Who handles what in West Islip
- Building and electrical permits: Town of Islip
- Sewer: Southwest Sewer District
- System approvals and grants: SCDHS, 631-852-5700 (permits) / 631-852-5811 (grants)
Frequently asked questions
Is West Islip on public sewer?
Mostly: about 91% of West Islip's land is inside Southwest Sewer District, but not every home inside a district is connected.
Can West Islip homeowners get septic grants?
Yes, if the home isn't sewered and isn't in a proposed sewer district: up to $20,000 from Suffolk and up to $25,000 from the state.
County rules, licensing and grant steps: Town of Islip overview, septic grants, replacement rules. More in Islip: Bay Shore, Islip.
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 Sanitary Code, Article 6 (amended 12/1/2020)
- Suffolk County: Septic Improvement Program information
- Suffolk County: SIP property eligibility lookup
Need cesspool or septic work in West Islip?
Submit a request with your address. We match it to an independent, licensed Suffolk County operator who serves that address; the operator confirms the time and price with you.
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