Septic Feasibility Before You Buy Land in CNY
Find out if a lot can hold a code-compliant septic system, and what kind, before you close. Soil survey screen, test pits and perc, read by a New York licensed PE.
Soil. Setbacks. Answer.
What a buyer learns from a septic feasibility check.
The NYS DOH design handbook says a field evaluation of the site and soil should be done before a property is bought or developed, and that the house and well should not be started until the septic system has been located and designed. On land without public sewer, the soil and setbacks decide whether a house can be built and what the septic system will cost.
No. The NYS DOH handbook says USDA soil survey maps help with general soil and wetness characteristics but are not a substitute for an on-site soil investigation. It is a good first screen before paying for test pits.
Appendix 75-A Table 2 requires 100 feet from a well to an absorption field, raised system or mound, 150 feet to a seepage pit, and 50 feet to a septic tank. If the system is upgrade and in the direct path of surface drainage to the well, the distance is 200 feet. Well distances go up 50 percent when water enters the well less than 50 feet below grade.
Appendix 75-A Table 2 requires 100 feet from an absorption field, raised system or mound to a stream, lake, watercourse or wetland, measured from the mean high water mark, and 50 feet from a septic tank. Some watersheds add more. In the Skaneateles Lake watershed, 10 NYCRR 131.1 requires the absorptive part of every system to be at least 100 feet from the lake or a watercourse.
Appendix 75-A says areas lower than the 10 year flood level are unacceptable for onsite systems, and the handbook says 100 year flood plains should also be avoided or handled with special construction. Onondaga County's design guidelines require all sewage disposal systems to be above the 100 year flood level, with elevations documented on the plan.
Appendix 75-A says slopes over 15 percent are unacceptable. The handbook's site table lists 0 to 15 percent for conventional trenches (up to 20 percent with extra spacing and soil depth), 0 to 12 percent for mounds and 0 to 8 percent for absorption beds.
Cayuga County says realty subdivisions of 5 or more lots of 5 acres or less must be approved. County fee schedules list subdivision review fees, for example Onondaga County at $250 plus $124 per lot in 2026 and Oswego County at $250 plus $45 per lot.
It depends on the bedroom count and the perc rate. A 3 bedroom house on soil that percs at 21 to 30 minutes per inch needs 275 feet of 2 foot wide trench under Appendix 75-A, and the regulation asks for an extra 50 percent of useable area to be set aside for future replacement whenever possible, all inside the well, water and property line setbacks.
Check the land before you buy it
On rural Central New York land, the septic system decides more about a lot than the price per acre does. A parcel with a wet spring water table, shale at 2 feet, a neighbor's well 60 feet from the only flat spot, or a wetland across the back can need a mound costing several times a conventional field, or have no legal spot for a system at all.
A lot septic feasibility check answers one question before closing: can this lot hold a code-compliant septic system for the house you plan, and what kind? It starts with a desk screen and ends with test pits and perc tests on the ground, read by a New York licensed Professional Engineer.
What the state says about checking first
The NYS DOH Residential Onsite Wastewater Treatment Systems Design Handbook is direct about it. It says septic location should be part of the first home site layout, not an afterthought, and that building the house or drilling the well should not start until the system has been located and designed. It says a field evaluation of the site and soil should be done before purchase or development.
The handbook also says onsite systems should only be used where public sewer is not feasible or available. If the lot sits along a sewer line, check with the town or sewer district first.
Step 1: the desk screen
- Soil survey. The USDA NRCS Web Soil Survey maps soil series for most of New York. The handbook says these maps help show general soil and wetness characteristics but do not replace an on-site investigation. Onondaga County's design guidelines call the county soil survey one of the most valuable sources for spotting high groundwater and bedrock.
- Flood level. Appendix 75-A says areas lower than the 10 year flood level are unacceptable for onsite systems. Onondaga County requires systems to sit above the 100 year flood level, with the elevation shown on the plan.
- Slope. Appendix 75-A says slopes over 15 percent are unacceptable. Mounds are limited to 12 percent and absorption beds to 8 percent in the handbook's site table.
- Wetlands and water. Appendix 75-A defines a wetland for design purposes as an area designated by DEC or another agency with jurisdiction. Onondaga County's guidelines note that a DEC permit may be required for construction within the 100 foot adjacent area of a state freshwater wetland. Streams and lakes are measured from the mean high water mark.
- Wells, yours and the neighbors'. Table 2 setbacks apply to any well or suction line, not just the one you will drill. Neighboring wells and septic systems along a shared line can block the best part of a lot.
- Watershed rules. Skaneateles, Otisco and Owasco lakes are public water supplies with their own rules on top of Appendix 75-A (details below).
Step 2: the setbacks that shape the lot
Appendix 75-A Table 2 minimum separation distances, in feet:
| Component | Well or suction line | Stream, lake, watercourse or wetland | Dwelling | Property line |
|---|---|---|---|---|
| House sewer (watertight) | 25 if cast iron, 50 otherwise | 25 | 3 | 10 |
| Septic tank | 50 | 50 | 10 | 10 |
| Distribution box | 100 | 100 | 20 | 10 |
| Absorption field | 100 | 100 | 20 | 10 |
| Seepage pit | 150 | 100 | 20 | 10 |
| Raised system or mound | 100 | 100 | 20 | 10 |
The notes to the table matter on tight lots:
- If the system is upgrade and in the direct path of surface drainage to a well, the closest part must be at least 200 feet from the well.
- Well distances go up 50 percent when aquifer water enters the well less than 50 feet below grade.
- Every part of the system must be at least 10 feet from any water service line.
- For systems built with fill (raised systems and mounds), distances are measured from the toe of the fill slope, which makes the footprint bigger than the field itself.
- Distances are also measured from the edge of the reserve area. Appendix 75-A asks for an extra 50 percent of useable area to be set aside for future expansion or replacement whenever possible.
The handbook also recommends keeping the field at least 50 feet from an embankment or very steep slope and at least 35 feet from a swimming pool.
Step 3: test pits and perc tests
Once a lot passes the desk screen, the real answer comes from digging. A deep hole test at least 6 feet deep in the likely field area shows the soil layers, mottling (seasonal high groundwater) and depth to rock. At least two perc tests measure how fast the soil takes water. Together they place the lot in a system category:
- 4 feet or more of useable soil, perc 1 to 60 min/in: conventional trenches are possible. See conventional septic design.
- 2 to 4 feet: shallow trenches (2 feet) or cut and fill (3 feet) may work.
- 1 foot of useable soil: a raised system or mound may work. Mounds can be designed on in-place soil up to 120 minutes per inch.
- Less than that, or faster than 1 min/in without blending: the standard options in Appendix 75-A run out, and the lot needs a closer look before you commit.
Spring matters. The handbook puts the normal high groundwater period at March 15 to June 30. A pit dug in a dry August can look better than the same ground in April, which is why the designer reads mottling, not just whether water is standing in the hole.
Watershed and county rules buyers should know
- Skaneateles Lake watershed. Under 10 NYCRR 131.1, new systems must be designed by a design professional, all site and soil appraisals must be witnessed by a City of Syracuse watershed inspector, and the absorptive part of every system must be at least 100 feet from the lake or a watercourse. Soil analyses should be done between about March 15 and June 30.
- Otisco Lake watershed. Under 10 NYCRR 131.50, inspections of sewage systems serving year round homes may not be more than three years apart, and seepage units may not be within 50 feet of the lake or a watercourse. Onondaga County sends Otisco plans to the Onondaga County Water Authority for review.
- Cayuga County, all of it. The county sanitary code requires every septic system to be inspected by a Cayuga County certified inspector before or at property transfer, and the tank pumped by a DEC licensed hauler unless it was pumped in the last 12 months. The county says the transfer inspection is valid for one year. In the Owasco Lake and Little Sodus Bay watersheds the code adds routine inspections: every 2 years for lakefront, every 3 years within 500 feet, and every 5 years elsewhere in the watershed. The county's Owasco watershed rules page lists the 1984 rules plus a proposed rewrite, with state recommendations dated 9/15/2023.
- Oswego County. The Health Department offers a septic dye test for realty, run over a 4 day period with written results, for $230.
- Subdividing. Cayuga County says realty subdivisions of 5 or more lots of 5 acres or less must be approved. Subdivision review fees are listed on the cost page.
Problem and solution: common lot findings
The only flat spot is near the neighbor's well. The 100 foot well setback is fixed for new construction, and a reduction needs a specific waiver from the health department. The design may move the field to a sloped area with a pressure distribution layout. See small lot setbacks.
Wet in spring. Mottling high in the pit points to a raised system or mound, plus surface water diversion. See wet yard or wetland.
Lakefront. Watershed rules, 100 foot water setbacks and narrow lots stack up. See lakefront septic.
Steep wooded lot. Over 15 percent is out under Appendix 75-A. See steep slope septic.
Timeline
The desk screen is quick. Test pits depend on frost and, where required, a witness. In the Skaneateles watershed the soil work should fall in the March 15 to June 30 window. Build the feasibility check into the purchase contract timeline so the result is in hand before the contingency runs out. The full permit sequence after purchase is on the septic engineering process page.
What Backwell does
Backwell pulls the soil survey and flood and setback screen, digs the test pits, sets up the perc tests, and hands the results to the PE for a written feasibility read: which system type fits, where it goes on the lot, and the rough footprint including reserve area. If the purchase goes ahead, the same data feeds the stamped design and the install.
Designs are prepared and stamped by a New York licensed Professional Engineer (PE). Backwell coordinates the design, the health department submittal, and the install.
Sources
- NYS DOH, Residential Onsite Wastewater Treatment Systems Design Handbook (includes Appendix 75-A and Fact Sheet #1)
- NYS DOH, 10 NYCRR Appendix 75-A, Wastewater Treatment Standards: Residential Onsite Systems
- USDA NRCS Web Soil Survey
- Onondaga County Health Department, Design Guidelines for Individual Sewage Systems
- 10 NYCRR 131.1, Skaneateles Lake watershed rules and regulations
- 10 NYCRR 131.50, Otisco Lake watershed rules and regulations
- Cayuga County Sanitary Code
- Cayuga County Health Department, Septic System Installation and Inspection
- Cayuga County, Owasco Watershed Rules and Regulations
- Cayuga County, Realty Subdivision Review
- Oswego County Health Department, Water and Septic Testing (septic dye test)
- Onondaga County Local Law 11 of 2025, Health Department fees (effective 1/1/2026)
- Oswego County Health Department, 2026 Plan Review Fee Schedule
