Can your lot take an ADU? Site, utilities and foundations

Where on your lot can an ADU go?

Three zoning numbers set the box your ADU has to fit inside. Zoning is your town’s rulebook for what can be built where.

• Setbacks are the minimum distances from each property line, and sometimes from the main house.
• Lot coverage is the share of your lot that buildings may cover. The ADU’s footprint usually counts.
• Height limits are often lower for an ADU than for the house.

Your town, village or city sets these numbers, not the state. Our guide to checking your town’s rules shows where to find them. The building code adds one more near the line: in the national model code New York’s builds on, a wall within 5 feet of a property line generally has to resist fire (ICC, 2024). Ask your town’s code office, the local office that handles zoning and building permits, for New York’s version.

Easements, slope and soil

An easement is someone else’s legal right to use part of your land, such as a utility line or a shared driveway. You generally can’t build on top of one. A current survey shows recorded easements, so get one before you pick a spot.

Slope adds digging, retaining walls and drainage work. It also limits septic, because New York won’t accept slopes over 15% for an on-site system (10 NYCRR 75-A.4, 2010). The free USDA Web Soil Survey gives a first look at soil type, drainage and depth to bedrock, though it’s no substitute for test holes dug on your lot.

Can a truck and a crane get in?

Every ADU needs a way in for concrete trucks and excavators, and modular units set by crane need more. One industry guide lists these minimums for the route to the site (National Modular Housing Council, undated):

If a crane or its load could come within 20 feet of a power line, OSHA requires extra steps, such as staying clear or having the utility shut the line off (29 CFR 1926.1408, 2026). Have the crane company walk your site before you sign.

Is the spot in a flood zone or near a wetland?

Check your address on FEMA’s Flood Map Service Center and New York’s DEC Environmental Resource Mapper.

In New York: building in a mapped flood hazard area takes a floodplain development permit from your town. The lowest floor must sit at least 2 feet above the base flood elevation, the flood height shown on FEMA’s map, or at least 3 feet above the highest nearby ground where the map gives no elevation (NYS DEC, 2026). A regulated floodway also needs an engineering study showing no rise in flood levels (NYS DEC, 2026).

If a federally backed or regulated loan pays for an ADU in a FEMA special flood hazard area, federal law requires flood insurance (FEMA, 2026). A septic system can’t go below the 10-year flood level (10 NYCRR 75-A.4, 2010).

In New York: since January 1, 2025, the state regulates any wetland that meets the legal definition, mapped or not. That generally means at least 12.4 acres, or one of 11 “unusual importance” tests, dropping to 7.4 acres on January 1, 2028, plus a 100-foot buffer. Not sure? Ask DEC for a jurisdictional determination, a written answer it has 90 days to give (NYS DEC, 2026).

What will the ADU sit on? Frost depth and foundations

Wet soil swells as it freezes, and it can lift, or “heave,” a foundation that isn’t deep enough. So footings, the base a building rests on, either go below the frost line or get insulated so the ground under them stays unfrozen. Perinton and Chili in Monroe County require 42 inches (Town of Perinton, 2025; Town of Chili, undated), and the Town of Ithaca lists 48 inches (Town of Ithaca, 2026). Ask your code office for yours.

A frost-protected shallow foundation wraps the slab’s edge in foam so the soil below doesn’t freeze. New York allows it only for buildings kept at 64°F or warmer, never for unheated space (2025 Residential Code of New York State, via UpCodes). On lots with fill, clay, wet spots or rock, a soils report from a geotechnical engineer is cheap insurance. Our winter-ready ADU guide says more about cold-weather foundations.

Water and wastewater: sewer, septic or a well

Public water and sewer

Many ADUs branch off the main house’s water line, while some water suppliers want a separate service and meter. The Monroe County Water Authority charges $3,350 for a new 1-inch service and meter on an existing main (MCWA, 2026). Have a plumber check that an older line can feed two kitchens and two showers.

Monroe County requires a permit for any new sewer connection and keeps roof drains and sump pumps out of the sanitary sewer (Monroe County Sewer Use Law, 2015). Towns add fees. Perinton lists $1,000 per unit for its sewer district plus $250 for the county hookup (Town of Perinton, 2026). Ask whether your ADU counts as a new unit, and whether it needs a pump to reach the sewer.

Septic: New York sizes it by bedrooms

In New York: new septic systems are designed for 110 gallons per bedroom per day (10 NYCRR 75-A.3, 2010). The smallest tank allowed is 1,000 gallons for up to three bedrooms, 1,250 gallons for four and 1,500 gallons for five (10 NYCRR 75-A.6, 2025). The rules were revised effective September 24, 2025 (NYS Department of Health, 2025), so older advice about your system may be out of date.

As an illustration only, a three-bedroom house plus a one-bedroom ADU on one system would be designed as four bedrooms: 440 gallons a day and at least a 1,250-gallon tank (10 NYCRR 75-A.3; 10 NYCRR 75-A.6). A tank and field built for three bedrooms might need to grow. Whether the health department counts the bedrooms together or wants a separate system is its call.

A septic field also needs at least 4 feet of usable soil above rock and seasonal high groundwater, at least two percolation (soak-in) tests, and these minimum distances (10 NYCRR 75-A.4, 2010):

On small lots without sewer, these distances often decide whether a detached ADU fits. One 2025 change helps: after a maintained “enhanced treatment unit,” which cleans wastewater more than a plain tank, the field can be about one-third shorter (10 NYCRR 75-A.6, 2025).

A private well

New York’s Health Department says a well that reliably gives 5 gallons per minute meets most homes’ needs. Weaker wells can work with added storage, and wells under half a gallon per minute aren’t recommended (NYS Department of Health, 2014). That guidance assumes one household, so get yield and water-quality tests and have a well professional confirm the well can serve both homes. Sharing a well doesn’t make it a public water system, which starts at 5 service connections (10 NYCRR 5-1.1, 2026), but lenders or your county may have their own rules.

Power, meters and heat

Most homes have 100-amp or 200-amp electrical service, and older or smaller homes are more likely to have 100 amps (Lawrence Berkeley National Laboratory, 2024). An ADU adds a kitchen, hot water, heating and cooling. An electrician runs a load calculation to see if your service can carry it, and one method, using your real peak demand from a smart meter, sometimes shows no upgrade is needed (same source). A full or outdated panel points toward an upgrade, which needs a permit and usually the utility.

The ADU can run off a subpanel fed from your main panel, with one bill for both homes, or get its own utility service and meter, which costs more but makes renting simpler. Around Rochester the utility is usually RG&E; elsewhere upstate it may be NYSEG or National Grid. Call early.

In New York: billing a tenant through your own submeter, a meter behind the utility’s, needs state Public Service Commission approval first, which generally takes about 6 to 8 months (NYS Department of Public Service, 2026).

Heat is the other big electrical question.

In New York: New York enacted restrictions for covered new construction, with exceptions for alterations, additions and changes of use to buildings existing before the applicable prohibition (NYS Energy Law §11-104). As checked October 9, 2026, the Department of State says the adopted fossil-fuel provisions remain suspended by court order; other 2025 code provisions remain in effect (DOS notice). We have not confirmed when the suspension will end. Ask your code office which current rules apply to your project before choosing equipment.

Ask your code office which current rules apply before choosing heating, hot-water and cooking equipment. And don’t count on a rebate: the state’s Clean Heat program says new construction can’t get its air-source heat pump space-heating incentives, though ground-source heat pumps and heat pump water heaters can qualify (NYS Clean Heat, 2026).

Who do you call for what?

In New York: whoever digs must call 811 at least 2 and no more than 10 working days ahead (16 NYCRR 753-3.1, 2026). The call is free, but utilities don’t mark private lines, such as the wire to your garage or the pipe to your septic tank (UDig NY, 2026).

What this means for you

A survey, a soil lookup, the flood and wetland maps, a call to the town and a septic or well check answer most of the big questions. They cost little next to designing a unit that can’t be built, so do them before you pay for plans.

Around here, the usual deal-breakers are septic capacity on lots without sewer, wetland buffers, flood zones, tight crane access and an electrical service too small for two homes. Each has a fix, such as a bigger septic system, piers, a smaller unit or a service upgrade, and each fix costs money.

Nothing here is approval to build. Your town, the health department and the utility make those calls, and an engineer, septic designer or lawyer should answer what this page can’t. Our property checker asks the first questions, and permits and the process covers what comes next.

A site-readiness checklist

A “no” or “not sure” is a question for a professional, not a dead end.

• Zoning allows an ADU here, and I know the setback, height, size and coverage limits.
• I have a current survey showing lot lines, easements, the well and the septic system.
• I checked the FEMA flood map and the DEC mapper.
• I know my soils, my town’s frost depth and whether the spot needs grading or piers.
• A truck, an excavator and, for modular, a crane can reach the spot without crossing the septic field or the well.
• On a well: recent yield and quality tests say it can serve both homes.
• On sewer: I know the fees. On septic: the health department will allow the extra bedrooms, and the spot keeps its distances.
• An electrician has done a load calculation, and I’ve called the utility.
• Heating options and electrical-service capacity have been reviewed, and I’ve asked the code office which current rules apply.
• Roof water won’t end up in the sewer, the septic system or a neighbor’s yard.
• My builder will call 811 and find private lines before anyone digs.

General information, not legal, tax or financial advice. Your municipality decides what can be built, and a professional should review decisions about money and contracts.