Short answer: Central Islip is split: about 23% of its land is inside Southwest Sewer District and the rest relies on cesspools, septic tanks and I/A systems.
Central Islip in numbers
| Town | Town of Islip |
|---|---|
| Area | 7.15 sq mi |
| In active sewer district | 23% · Southwest Sewer District |
| In proposed district | 2% · Central Islip |
| Groundwater zone | Zone I 61%, Zone VII 39% |
| FEMA flood zone | 2% |
| One-family lots / median | 5,710 / 0.23 ac |
| One-family lots outside active sewer districts | about 5,418 |
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
Central Islip is split: about 23% of its land is inside Southwest Sewer District and the rest relies on cesspools, septic tanks and I/A systems. A sliver (2%) is inside a proposed district (“Central Islip”). Counting lots instead of land: about 5,418 of the 5,710 one-family lots touching Central Islip lie outside any active sewer district, and those homes rely on on-site systems. Neighbouring areas: Islandia (8% sewered, median lot 0.27 ac); North Great River (1% sewered, median lot 0.46 ac); Islip Terrace (69% sewered, median lot 0.25 ac). By active-sewer share Central Islip ranks 24 of the 165 Suffolk hamlets and villages we measured.
Groundwater, lots and flood exposure
Groundwater management Zone I (61%) and Zone VII (39%): 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 5,710 one-family lots touching Central Islip, with a median of 0.23 acres (about 9,900 sq ft), below that minimum and the 139th-largest median lot in our Suffolk table. 2% of the land is in FEMA's 1%-annual-chance flood zone.
For most Central Islip homes the main running cost is pumping; when a cesspool fails, the replacement must include at least a septic tank, and grants can cover most of an I/A system.
Who handles what in Central 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 Central Islip on public sewer?
Partly: about 23% is inside Southwest Sewer District; the other 77% relies on cesspools, septic tanks and I/A systems.
Can Central 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: Islandia, Brentwood, 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 Central 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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