A buyer touring a two-acre lot off Glen Road assumes the septic system already passed some kind of county checkpoint. It hasn't, and in Montgomery County, it may never have to.
That surprises people who came to Potomac from a Bethesda condo or a Chevy Chase rowhouse, where water and sewer are just utility bills. Several Maryland counties, including Anne Arundel, Baltimore, Carroll, Frederick, Harford, and Queen Anne's, require a certified septic inspection every time a property changes hands. Montgomery County does not. There is no county-run inspection gate that a Potomac sale has to clear. The system that treats a household's wastewater, and the well that may supply its drinking water, gets evaluated only if the buyer arranges it, the lender requires it, or the seller volunteers the history. For a region where a single-family lot routinely trades in the seven figures, that gap is worth understanding before it becomes a renegotiation on day thirty of a contract.
Why some of Potomac's best dirt was never wired for public service
Montgomery County classifies every parcel into a water and a sewer service category, and the classification tells you something the listing photos never will. Category 6 covers land the county's Water and Sewer Plan has decided will never receive community water or sewer, not in the current ten-year plan and not beyond it. The county's own environmental protection page is direct about it: development in those areas is expected to use private wells and septic systems, full stop. A meaningful share of Potomac's large-lot inventory, the multi-acre parcels that give the area its horse-country feel, falls into that designation. It isn't a temporary gap waiting on a capital project. It's a permanent feature of the land.
That single fact reshapes how a buyer should think about the lot itself. A Bethesda kitchen renovation worries about permits and finishes. A Potomac teardown or major addition worries about something underground first: what the septic field can support, whether the well yields enough water for the household, and how much buildable footprint remains once setbacks from the septic system, the well, and any forest conservation easement are subtracted from the parcel's total acreage.
The land is worth what the septic field will allow it to become
A local design-build firm that has worked lots in Bethesda, Kensington, and Potomac since 2007, Colonial Design & Build, put the math plainly on their own site: a dated rambler on two acres can trade near a million dollars with almost all of that value sitting in the ground, not the structure. Put a new 5,000 to 7,000 square foot house on the same parcel and it competes with anything else in the 20854 zip code, because new construction on real acreage is the one thing Potomac buyers consistently can't find on the open market.
Here's the part that changes an offer strategy. That land value is not open-ended. Septic capacity is tied to a percolation test and an approved bedroom count on file with the county, and the firm is explicit that they confirm what the county will actually permit before design starts, not after a buyer has fallen in love with a six-bedroom floor plan the lot can't support. A buyer who skips that step is pricing the dirt on square footage and hope. A buyer who pulls the septic permit history first is pricing it on what the county will actually allow, which is the number that determines resale value years later.
Some of that homework has real dollar figures attached, and they're worth knowing before you write an offer. Under Maryland's nitrogen-reduction requirement, a full Best Available Technology septic installation for a typical three to four bedroom home runs $20,000 to $40,000 or more. If the soil doesn't perc conventionally and the lot needs a sand mound or advanced treatment system, that number climbs to $30,000 to $45,000 and up. A standard drilled well runs $8,000 to $15,000. None of that is disqualifying on a parcel where the land alone can approach seven figures, but it belongs in the offer math, not discovered after ratification.
What the state actually requires you to say
Maryland doesn't leave septic condition to chance, even without a county-mandated inspection. Every seller of residential property completes the state's Residential Property Disclosure and Disclaimer Statement under Section 10-702 of the Real Property Article, and the septic section asks a direct question: is the system functioning properly, yes, no, or unknown, and when was it last pumped. The water supply line asks whether the property is on public water, a private well, or something else. Sellers can choose the disclaimer path and sell as-is, but even then, known latent defects, meaning hidden problems that threaten health or safety, still have to be disclosed. A septic system that's been failing quietly doesn't stop being a latent defect just because the seller checked the as-is box.
This is the real difference between Montgomery County and its neighbors that mandate inspection. In Anne Arundel or Frederick, a licensed inspector produces a report before closing regardless of what either party knows or wants to say. In Potomac, the honesty on that disclosure form and the buyer's own due diligence are doing the work a county inspector does elsewhere. That's not a loophole. It's a different allocation of responsibility, and it rewards sellers who get ahead of it and buyers who ask the right questions before they're under contract with a deadline attached.
What I ask sellers to do before we list
I've built my practice around the idea that thoughtful preparation changes outcomes more than a fresh coat of paint ever will, and a Potomac well and septic system is exactly the kind of detail that rewards preparation. Before we put a large-lot property on the market, I want the septic tank's last pump date documented, the permit on file with the county pulled so we know the approved bedroom count, and the well tested for yield and water quality if it hasn't been checked recently. None of that is required by Montgomery County. All of it removes the single most common reason a Potomac contract gets renegotiated in week three, which is a buyer's inspector finding something the seller genuinely didn't know, or worse, something they hoped wouldn't come up.
A seller who walks into a listing with that paperwork in hand isn't just protecting the deal. They're answering the disclosure form with confidence instead of guesswork, and that confidence shows up in how a buyer negotiates.
What I ask buyers to check before they write an offer
If you're the one making the offer, ask for the septic permit and its approved bedroom count before you fall for a floor plan the lot can't legally support. Ask whether the well has been tested for yield in the last few years, not just water quality, since a low-yield well with a large household on it is a maintenance problem waiting to happen. Ask whether the property sits inside a forest conservation easement, since Montgomery County's Forest Conservation Law applies to tracts of 40,000 square feet or larger, and Potomac has no shortage of lots that size. And ask, plainly, whether the property is on public water with private septic, private well with public sewer, or fully private on both, because those three situations carry very different maintenance obligations and very different resale ceilings.
A few questions worth asking directly
Is a septic inspection required to sell a home in Potomac? No. Montgomery County has no law requiring a septic inspection at the time of sale, unlike Anne Arundel, Baltimore, Carroll, Frederick, Harford, and Queen Anne's counties. Sellers still must complete Maryland's disclosure form, which asks about septic function and last pump date, and any known latent defect must be disclosed regardless of which disclosure path is chosen.
Can a Potomac property have public water but a private septic system, or the reverse? Yes. Montgomery County tracks water and sewer categories separately for every parcel, and a property can be classified differently for each. It's common in Potomac to find a home on public water with a private septic system, particularly in Category 6 areas where public sewer was never planned.
How long is a Montgomery County septic permit valid if I need to install or repair a system? Twelve months from issuance, with one allowed extension of six months. After that, a new application and fee are required, which is worth building into any timeline for a teardown, rebuild, or major addition on a septic-served lot.
Potomac rewards the buyers and sellers who do the underground homework before the floor plan gets drawn or the sign goes in the yard. If you're weighing a purchase on acreage, or preparing a long-held family property for market, I'd rather walk the lot with you and pull the right records now than watch a great contract get renegotiated over something we could have found in week one. Laurie Rosen would love to help you think it through. Let's Connect.
