Two colonials sit a mile apart. Both list at $2.85 million, both show four bedrooms and a finished lower level in the marketing copy, both went under contract in the same May 2026 week when Houlihan Lawrence recorded a median single-family sale of exactly that number. One buyer will finish the attic next spring and add a pool in year three. The other buyer will discover, roughly forty-five days after closing, that neither project is permittable on the lot as it sits.
Nothing in the MLS sheet distinguishes them. The distinction lives in a state health-code section written in 1998 and in the fact that New Canaan's sanitary sewer system covers only the more central sections of town, with 30-plus miles of pipe and 900-plus manholes. Everything outside that footprint runs on private septic, and every private-septic property is quietly governed by a rule that most buyers meet for the first time when their contractor asks them for a soil report.
The Rule That Sits Under Every Non-Sewered Lot
Connecticut Public Health Code Section 19-13-B100a became effective on August 3, 1998. The revised code makes clear that testing information is required prior to approval for building conversions, additions, and construction of accessory structures, even if the proposed construction is a non-water-use structure such as a detached garage, shed, deck, or in-ground pool, because whenever a portion of the property will be lost to the new structure, a code-complying area for a future replacement septic system must be found exclusive of the proposed construction area before approval is granted.
The New Canaan Health Department applies it plainly. Any addition to the house, even one that does not change the footprint, will probably require testing for a code-compliant septic system, including finishing the attic or basement, and improvements such as swimming pools, garages, pool houses, or sheds may also require testing under Section 19-13-B100a. The town's own guidance goes a step further: if the home is on septic and the owner plans to remodel, add on, or build an accessory structure such as a barn or pool, a code-complying reserve septic plan must be on file with the New Canaan Health Department prior to submitting the building permit.
Read that sentence as a buyer. The permit will not issue against the house you are buying. It will issue against a diagram of a hypothetical replacement septic system that has to fit somewhere on your lot, at code setbacks, without conflicting with the pool or addition you actually want.
What The Sewer District Actually Covers
The sewer map is not intuitive. It follows the historic village core and radiates from there.
- Sanitary sewer, per the town engineer, was first installed in 1907 and now consists of 30-plus miles of pipe and 900-plus manholes serving only the more central sections of New Canaan.
- Whether a home is on the town's sewer line or on a septic system, everything ends up at New Canaan's wastewater treatment plant on the Fivemile River, built in the 1960s with a major upgrade in 1999.
- Inside the district, residential sewer users pay a tiered flat fee. R Category 1 applies to homes and condos with assessed value under $450,000, R Category 2 covers $450,000 to $749,999, and R Category 3 covers $750,000 or more. Most single-family owners land in Category 3.
- Outside the district, everything the house discharges runs to a private tank and leach field, and the state's separation rules bind the lot. New Canaan's own ordinance is explicit: except as permitted under the Connecticut Public Health Code, no septic system shall be constructed less than 75 feet from any well used for drinking water for human beings.
Buyers who tour a house with a well in the front yard and a leach field behind the garage rarely walk the 75-foot radius from the wellhead. The state does, and so does the town sanitarian.
Reading The May 2026 Numbers Through This Lens
The Houlihan Lawrence single-family report for May 2026 shows a market that has loosened enough for buyers to actually price this friction in.
| Metric (New Canaan, all SFH) | May 2026 | vs. May 2025 |
|---|---|---|
| Median sale price | $2,855,000 | -4.8% |
| Average sale price | $3,221,890 | +1.0% |
| Units sold | 11 | -31.3% |
| Average days on market | 52 | -29.7% |
| Sale-to-list ratio | 98.47% | -5.0% |
| Months of inventory | 3.60 | +24.1% |
Twelve months earlier, the town was closer to a two-month supply market where sale-to-list ratios ran above 103% and diligence contingencies got shortened or waived. At 3.6 months of inventory and 98.47% of list, a buyer has room to make an inspection contingency mean something. That room is where the B100a question stops being a nice-to-know and becomes a line item in your offer.
The listing side agrees the market has cooled. In July 2026, New Canaan homes were listed at a median price of $2.59 million and spent a median of 28 days on the market, a modest decrease from July 2025. Time on market is a proxy for negotiating leverage, and leverage is what turns a due-diligence finding into a price adjustment.
The Option Value You Are Actually Buying
Two identically priced homes are not identically priced when you value them the way an underwriter values a bond with an embedded option.
Home A is inside the sewer district. Wants to finish the third floor into a primary suite? A structural review and a bedroom-count check with the health file. Wants a pool? A zoning setback question and an inland-wetlands check if the yard slopes toward a watercourse. Renovation optionality is broad.
Home B is on a two-acre wooded lot outside the district. It shows a small backyard clearing, a leach field marked on an old as-built, and a well between the driveway and the road. Same list price. The B100a rule says any addition, pool, garage, or finished basement conversion has to leave room for a full code-complying replacement system on that lot. If the licensed engineer's soil testing finds ledge, seasonal high water, or a slope that will not perc, the town sanitarian will not approve the building permit, and the health department has said as much on its own guidance page.
Home B is not worth less than Home A in every case. It may be worth more. It might already have a documented, engineered reserve area on file that gives its next owner years of permitting cover. The only way to know is to check.
A Due-Diligence Sequence For The Health Department File
Robin's approach on non-sewered listings runs in this order, and it is the order a buyer's agent should follow whether they are working with our office or with anyone else.
- Pull the property file. Health department files are always available for viewing on the public computers outside the Land Use Offices and digitally on the town website via "documents on demand" under agenda and minutes, with a direct link on the Health Department page to all scanned property files.
- Confirm whether a B100a reserve plan already exists. If it is unclear whether the property has a code-complying reserve septic plan on file, the Health Department at (203) 594-3037 will provide guidance on next steps.
- Order a septic inspection tied to the transaction, not to routine maintenance. A transfer-inspection requirement applies to 23 Connecticut municipalities concentrated in Fairfield and New Haven counties, and Greenwich, Stamford, Darien, and New Canaan all require comprehensive septic inspections with written reports from licensed professionals.
- If the plan does not exist and the buyer intends any renovation, price the reserve-area study into the offer. Depending on scope and site limitations, the town may require the owner to hire a Connecticut-licensed professional engineer to conduct soil testing and submit a code-complying septic plan, and it is a real number and a real timeline.
- Confirm the well setback. Any past addition or pool built without a documented B100a review could be sitting on top of the very ground your future replacement leach field needs.
None of this shows up in a listing photo. All of it shows up in the closing timeline.
Two Sub-Markets, Two Clocks
The West and South of Town sub-areas that sit largely inside or adjacent to the sewer district are also the sub-areas nearest to two capital projects that keep coming up in relocation conversations: a state-of-the-art community center with aquatic and fitness facilities expected to complete in late 2026, and a $15 million upgrade to the New Canaan branch Metro-North station focused on parking and commuter facilities. Both projects are covered on Niang Real Estate's town overview and both are the kind of anchor that tends to pull entry-level buyers into the neighborhoods closest to them.
The Back Country and mid-country sub-markets, largely on septic and well, sit on a different clock. Their pricing turns less on downtown amenity delivery and more on the size, age, and engineered reserve of the on-site systems the buyer is inheriting. Two years ago the market moved too fast for that distinction to matter. In a 3.6-month-supply market with average days on market at 52, it matters again.
The thesis is narrow. The median sale price tells you what a New Canaan home closed for in May 2026. The sewer map tells you what the next owner can do with it.
FAQ
Does a home inside the sewer district avoid B100a entirely? Yes for the septic-reserve question, because there is no on-site septic system to preserve room for. Zoning setbacks, wetlands review, and the town's separate storm-sewer rules still apply, and the sewer pipe from the house to the main sewer line is the homeowner's responsibility to maintain, repair, or replace, which is a cost buyers often forget to underwrite.
Is a septic inspection the same as a B100a reserve-area determination? No. A septic inspection tells you whether the current system is functioning. A B100a review, driven by a licensed installer or engineer and witnessed by the Health Department, tells you whether a code-complying replacement system can be sited on the lot in the future. Renovation permitting hinges on the second question.
How long does a reserve-area study take? Timelines vary with soil conditions and town review queues, and seasonal ground conditions affect when testing can be witnessed. Building this into the inspection contingency, rather than assuming it will happen after closing, is the difference between negotiating on findings and living with them.
If you are weighing a New Canaan purchase against Darien or Westport, or comparing two homes in New Canaan whose price-per-square-foot looks identical on paper, the questions above are the ones that separate a strong offer from an expensive one. Robin Bartholomew reads the town by its infrastructure map, not its headline median, and would welcome a conversation about the specific lots on your short list. Schedule a Consultation.
