In Plymouth a septic inspection is really a question about counting. How many bedrooms does the approved design say, how many does the listing say, and how many does the house actually have once you’ve walked all the way through it. Those three numbers disagree here more often than they agree, and the gap between them lands squarely on whoever owns the place after closing.
Plymouth’s accessory dwelling unit ordinance is specific in ways that matter at a sale. One ADU per single-family lot. No more than two bedrooms inside it, no more than two adults living there, and a hard cap of 1,000 square feet of habitable floor area. The unit has to make provision for water supply and sewage disposal in compliance with RSA 485-A:38. That last clause is the one that gets skipped, because an in-law apartment is easy to frame and a good deal harder to permit.
The land math is just as firm. A lot on an on-site septic system needs 43,560 square feet — a full acre — for the first dwelling unit, and another 7,500 for each unit past it in the single-family and multifamily districts. Which means a house on a bare acre with a finished apartment over the garage is, on paper, short of land for what’s already standing on it. Sometimes there’s a permit behind that. Frequently there isn’t.
And the housing stock more or less invites it. Thirty percent of Plymouth’s single-family homes were built before 1939 — high ceilings, back stairs, and a great deal of unfinished space that somebody finished at some point across the last eighty years without calling anybody first. The town’s assessor counts 1,717 residential parcels carrying 2,344 dwelling units. Those extra units came from somewhere.
Get the NHDES approved design before the inspection rather than after. It takes a few days and it gives you the design load in bedrooms, which means we can walk in already knowing whether the house is running inside or outside its own paperwork — instead of finding out on the day and waiting another week to learn what it means.
Locate, pump, then look. The tank has to be empty before it tells you anything — full, all you can see is a water level. Empty, you can read the inlet and outlet baffles, the condition of the walls, whether the outlet is sound, and most importantly whether the field sends water back into the tank once it’s drawn down. Then we run water through the house, a real volume of it, because a lightly used system will show you nothing at all until it’s under a load.
Alongside that, pull the paperwork. NHDES holds approved designs for systems permitted in recent decades, and the design states the load in bedrooms. Match it against the listing. A three-bedroom design underneath a house being marketed as four, or as three plus an apartment, is a real finding with a real cost attached, and the inspection window is the only stretch where you have any leverage over it. After closing it is simply yours.
One Plymouth-specific check worth adding to the list: ask where the sewer district line runs. If the property sits close to the main, a marginal system is a far less frightening finding than the same result would be four miles up the road, because connection is a live option and the building code already leans that way. If it’s nowhere near, then the septic system is the only answer this property is ever going to have, and it ought to be priced accordingly.
A full pre-purchase inspection is $400–$700, with pumping billed separately if you want the tank emptied during the visit — and you do, because it’s the only way to see the baffles properly. If the house has an in-law unit or a finished apartment, say so when you book. It changes what we’re looking for and it changes what the paperwork has to show.
Whether it was ever permitted, and whether the septic design accounts for it. Plymouth allows one ADU per single-family lot, capped at two bedrooms and 1,000 square feet, and it has to provide for sewage disposal under RSA 485-A:38. An unpermitted unit sitting on an unmodified system is two problems rather than one, and both of them transfer to you at closing.
Generally no. New Hampshire requires a site assessment under RSA 485-A:39 at the sale of developed waterfront inside the protected shoreland, and that doesn’t reach most Plymouth properties. Everywhere else it’s a contingency you have to ask for. Nobody is obliged to hand you one, and nobody is obliged to volunteer what they already know.
Then the system predates the record, which is common in the pre-1939 stock here. It isn’t automatically bad news; plenty of those systems work fine. It does mean the inspection is the only information you are ever going to get, so do it properly — located, pumped, opened, and run under load — and budget as though replacement sits on a nearer horizon than you’d like.
It’s a reason to do it more carefully, not less. A rested system looks excellent, because nothing has gone through it in months. That’s exactly why we run water through the house during the visit. A marginal field shows itself under load and hides completely without one, and an empty-house inspection done without loading the system is close to worthless.
The occupancy you intend is the number that matters, not the one the seller had. A system that comfortably carried a retired couple can be badly undersized for five tenants. Tell us the plan before the visit and we’ll assess the system against that load and against what the approved design actually permits, which are two separate questions with two separate answers.
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