Is an I/A Septic System Required for My Property? (LI)

Short answer: In Suffolk County an I/A system is required when you build a new house, expand one in a way that changes the sanitary system or adds bedrooms beyond the approved count, or do a major reconstruction, for applications filed since July 1, 2021. Replacing a failed cesspool alone needs a septic tank and leaching ring, not an I/A system, except where a town such as East Hampton or Southampton requires more. Nassau has no county mandate we could find.

Last updated: November 2026. A decision guide, not a ruling. SCDHS (Suffolk) or your building department (Nassau) decides for a specific parcel.

Quick check

Where is the property? Is it connected to sewer? What are you doing? Town (Suffolk only)

Rules as of November 7, 2026. Confirm with SCDHS (631-852-5700) or your building department.

Suffolk County, case by case

1. Is the house on sewer, or inside an existing sewer district?

For construction projects inside an existing sewer district, Article 6 requires connection to the community sewer unless the developer shows it can't be arranged (§760-608.A). On-site rules, including I/A, then don't apply. Use the county's eligibility map or our town pages to see the districts near you.

2. New house, qualifying addition or major reconstruction: I/A required

§760-614.A: applications to the Office of Wastewater Management on or after July 1, 2021 must include an I/A system when the project is a new or expanded single-family residence or a major reconstruction that is permitted an on-site system. The code's definitions do the work:

New or expanded single-family residence covers a new house, an addition or modification that requires adding or modifying the sanitary system, a new accessory structure that requires it, and adding bedrooms beyond the number OWM previously approved.
Major reconstruction means rebuilding or modifying where the cost exceeds 50% of the existing structure's market value as set by a NYS-certified appraiser (land, landscaping and detached structures are excluded from both sides of the comparison).

3. Exemptions

Not a major reconstruction, and no bedrooms beyond the approved count (§760-614.D).
Not a major reconstruction, no change in square footage or footprint, and five bedrooms or fewer (§760-614.E).
Parcels inside a proposed sewer district approved by the Commissioner (§760-614.C).

Where bedrooms are added, a conforming conventional septic system is the minimum even if an exemption applies.

4. Replacing a failed or old cesspool with no construction

Since July 1, 2019 any replacement or retrofit needs SCDHS approval (your licensed liquid waste contractor files it), and the new system must meet current standards: at least a septic tank ahead of a leaching structure. No cesspool-for-cesspool swaps. The county was explicit that the change does not require upgrading a working system.

5. Town rules on top of the county

East Hampton: low-nitrogen system required for new buildings, any voluntary replacement of an existing sanitary system and substantial expansions (Town Code §210-3-2.B).
Southampton: in the high-priority area of the town's Water Quality Improvement Project Plan, I/A required for all new residential construction, any substantial upgrade SCDHS requires and a 25%+ floor-area increase; also when the Conservation Board or Environment Division requires a new system (§123-52).
Shelter Island: the town's rebate policy notes that the SCDHS or Town Code §43-10.4 may mandate an I/A upgrade as part of construction review.

We did not find town-wide I/A mandates in the other seven Suffolk towns as of November 7, 2026. Villages have their own codes; ask the village building department.

Nassau County

We found no county requirement. Nassau's health department reviews larger projects; single-family approvals generally go through the town, city or village. The S.E.P.T.I.C. grant is voluntary and eligibility depends on being outside existing and proposed sewer districts.

Selling or buying

No rule we found requires installing an I/A system when a Long Island house changes hands. New York sellers disclose the system type, age and last pump-out on the Property Condition Disclosure Statement. If an I/A system is already there in Suffolk, Article 19 sets 60-day notice and registration deadlines for seller and buyer. More in buying a home with a cesspool.

Frequently asked questions

Do I have to install an I/A system if my cesspool fails in Suffolk County?

Not under the county code alone. Since July 1, 2019 the replacement must meet current standards, which means at least a septic tank ahead of a leaching structure, and it needs SCDHS approval. An I/A system is optional (and grant-eligible) unless your town requires it: East Hampton requires a low-nitrogen system for any voluntary replacement.

Does adding a bedroom trigger an I/A requirement?

It can. In Suffolk, adding bedrooms beyond the number the Office of Wastewater Management previously approved makes the project a new or expanded single-family residence, and applications filed on or after July 1, 2021 must include an I/A system unless an exemption applies. Where bedrooms are added, a conforming conventional septic system is the minimum.

Is an I/A system required in Nassau County?

We found no Nassau County requirement for existing homes as of November 2026. Nassau's S.E.P.T.I.C. grant is voluntary. Ask your town, city or village building department whether a local rule applies to your project.

Do I need an I/A system to sell my house?

No county or state rule we found requires installing one at sale on Long Island. If the house already has an I/A system in Suffolk, the seller must notify SCDHS within 60 days of the transfer and the buyer must register it within 60 days.

Sources

Each source was re-read on November 7, 2026. Program amounts and rules change; the agency page is the authority.

Have a cesspool or septic question on Long Island?

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