How do you build an ADU for Finger Lakes winters?

What makes a Finger Lakes winter different?

The building code turns winter into a short list of local numbers. New York requires every city, town and village to publish its own design values, including ground snow load, frost depth and winter design temperature (19 NYCRR 1203.3(k), NY Department of State). Your designer and builder start from that list, which the code calls Table R301.2.

Here is one town’s, as an example. The values come from the Town of Ithaca’s 2025 table (Town of Ithaca, 2026):

Your town’s numbers may differ, so ask your town’s code office, the local office that issues building permits, for its table before anyone draws a plan.

In New York: the energy code puts most of the Rochester and Finger Lakes area, including Monroe, Ontario, Wayne, Livingston, Yates, Seneca and Tompkins counties, in climate zone 5, a band defined by how cold it gets. Much of the North Country and the Adirondacks is in colder zone 6 (2025 Energy Code of New York State, via UpCodes). The 2025 energy code took effect December 31, 2025, with no grace period (NY Department of State, 2025).

Snow, roof shape and ice dams

A roof has to be built for whichever is larger, the code’s standard roof load or your town’s snow load (2025 Residential Code of New York State, via UpCodes). That adds up fast. As a rough illustration, 40 pounds per square foot (Town of Ithaca, 2026) over a 24-by-26-foot footprint is nearly 25,000 pounds of snow, about 12.5 tons. Engineers figure real loads with roof shape, exposure and drifting, so read that only as a sense of scale.

Roof pitch, or slope, is written as rise over run, so a 2/12 roof rises 2 inches for every 12 inches across. A steeper roof isn’t automatically a stronger one, because the framing has to be designed for your snow load either way. Pitch does change the roofing rules. Asphalt shingles need a slope of at least 2/12, and between 2/12 and 4/12 they need a double layer of underlayment, the sheet under the shingles (2025 Residential Code of New York State, via UpCodes).

Where the snow goes matters as much as how much there is. Good winter design keeps a low ADU roof out from under a taller roof that sheds or drifts onto it. It keeps snow from sliding onto the front door, walkways, decks or the outdoor heat pump unit. Simple roof shapes and a covered entry help on every count.

Ice dams

An ice dam is a ridge of ice along the roof edge. Heat leaking into the attic melts snow on the roof, and the water refreezes at the cold roof edge, called the eaves, where it can back up under the shingles.

In New York: where your town’s table calls for it, the code requires an ice barrier, a waterproof layer under the roofing, from the roof edge to at least 24 inches inside the outside wall line. On roofs of 8/12 or steeper, it runs at least 36 inches up the slope from the edge. The exemption for detached accessory buildings covers only those with no heated or cooled space, so it doesn’t apply to an ADU (2025 Residential Code of New York State, via UpCodes).

The ice barrier is the backup, not the cure. The real fix is a cold, well-sealed attic: thick insulation and few air leaks between the living space and the roof.

Foundations frost can’t lift

Wet soil expands as it freezes and can heave, or lift, a footing (the concrete base a wall sits on) that isn’t deep enough. New York’s code gives four ways to protect a foundation: go below the local frost line, build a frost-protected shallow foundation, follow the ASCE 32 engineering standard, or bear on solid rock (2025 Residential Code of New York State, via UpCodes). The frost line is 42 inches in Perinton (Town of Perinton, 2025) and 48 inches in the Town of Ithaca (Town of Ithaca, 2026).

A frost-protected shallow foundation wraps rigid foam around the edge of a shallow slab so the ground beneath stays unfrozen. It saves digging. But New York allows it only for buildings kept at 64°F or warmer, and not for unheated porches, garages or rooms (2025 Residential Code of New York State, via UpCodes). If the ADU might sit empty and unheated over a winter, as a seasonal guest cottage might, plan a deeper foundation or commit to keeping the heat on.

The same section lets small free-standing accessory buildings skip frost protection: 600 square feet or less, light framing, eaves 10 feet high or lower (same source). Our reading is that this covers sheds and garages, not a place someone lives, so confirm with your code office before anyone counts on it. Our site and utilities guide compares foundation types.

Insulation, air sealing and the New York energy code

A few terms first. R-value measures how well insulation slows heat loss, and higher is better. U-factor measures how much heat a window lets through, and lower is better. Continuous insulation is a layer wrapped outside the framing so heat can’t slip out through the studs. Here are the climate zone 5 minimums under New York’s 2025 energy code (2025 Energy Code of New York State, via UpCodes):

A blower-door test uses a big fan in a doorway to measure how leaky the finished building is. The same code requires ducts in new buildings to run inside the heated space (same source).

A building that tight needs fresh air on purpose. When a home meets the energy code’s air-sealing rules, New York’s residential code requires mechanical ventilation (2025 Residential Code of New York State, via UpCodes). In a small ADU that is often a heat-recovery or energy-recovery ventilator (an HRV or ERV), which brings in fresh air and saves heat from the stale air going out. Plan a spot for it early.

Heat pumps or something else?

For a new ADU in New York, the likely answer is a cold-climate heat pump. A heat pump heats and cools by moving heat with electricity instead of burning fuel.

ENERGY STAR’s cold-climate label means a heat pump was tested down to 5°F (ENERGY STAR, 2026). Size the system for your town’s winter design temperature, which is 0°F in the Town of Ithaca’s table (Town of Ithaca, 2026). The Northeast Energy Efficiency Partnerships keeps a list of heat pumps with published cold-weather performance (NEEP, 2026). As common practice, the outdoor unit goes on a wall bracket or stand above the snow, away from roof drip and drifts, following the maker’s clearances.

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.

Don’t build the budget around a rebate. The state’s Clean Heat program says new construction isn’t eligible for its air-source heat pump space-heating incentives, though ground-source heat pumps and heat pump water heaters still can be (NYS Clean Heat, 2026). The federal credit of up to $2,000 a year for heat pumps covered only equipment placed in service before December 31, 2025 (IRS, 2026). An all-electric unit also adds load to your electrical service, which our site and utilities guide covers.

Freeze-safe plumbing

Where the winter design temperature is 32°F or lower, which includes the Town of Ithaca at 0°F, New York’s code says water and drain pipes outside the building, in exterior walls, attics, crawlspaces or any other spot that can freeze must be protected with insulation, heat or both. A water service pipe must also be at least 12 inches deep and at least 6 inches below the frost line (2025 Residential Code of New York State, via UpCodes; Town of Ithaca, 2026). With Perinton’s 42-inch frost line, that puts the water line at least 48 inches down (Town of Perinton, 2025).

A few habits keep pipes safe beyond the code minimum. These are common practice, not code rules:

• Keep supply pipes in interior walls. Kitchens and baths on outside walls can still work, with the pipes routed through the inside of the building.
• Keep the crawlspace inside the warm envelope. If pipes run there, it should be sealed and insulated, not vented to winter air.
• Plan the trench. A detached ADU’s water and sewer lines run underground from the house or street, below frost. On a big lot, a long run is a real cost. Local officials set how deep a sewer line to a septic system goes (same code source).
• Use frost-proof outdoor faucets, and know where the ADU’s shutoff valve is.
• Don’t leave it cold and full. If the ADU will sit empty, keep the heat on, which a shallow foundation needs anyway, or have a plumber drain and winterize it.

What this means for you

• Start with your town’s design table. Snow load, frost depth and winter design temperature drive the roof, the foundation and the heating system.
• Expect a tighter, thicker building. In zone 5 that means R-49 in the attic, walls well beyond bare studs, windows rated 0.27 or lower, a blower-door test and mechanical ventilation (2025 Energy Code of New York State, via UpCodes).
• Check the current heating rules. Ask your code office which rules apply and review heating options with your designer; a brand-new unit may not get the air-source rebate.
• Match the foundation to how the ADU will be used. An insulated shallow foundation only works in a building that stays heated.
• Think about where snow lands. Entries, paths and the heat pump should be out of the way of sliding snow, which matters even more for an older parent walking to the main house.

None of this is a design or an approval. Your town’s code office has the local numbers, and a licensed designer or engineer has to size the roof, foundation and heating for your actual site.

What to ask a builder

• What snow load, frost depth and winter design temperature are you designing to, and where did those numbers come from?
• Is the roof engineered for that snow load? Where will snow slide when it comes off?
• What foundation do you plan, and does it depend on the building staying heated?
• How will the building meet the energy code? Which wall insulation, which windows, and when is the blower-door test?
• How will fresh air get in, and where does the HRV or ERV go?
• Which heat pump is it, is it on the NEEP cold-climate list, and is it sized for our design temperature? Where will the outdoor unit sit?
• Where do the water lines run, how deep is the service line, and are any pipes in outside walls or unheated spaces?
• Beyond the ice barrier, how are you keeping the attic cold and sealed?
• Can my electrical service carry an all-electric unit, or will it need an upgrade?

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.