Utilities: What Do They Cost?!

Why the cheapest line on the flyer is never the whole number

Advertised ADU prices have a habit of describing the box and not the project. A unit price can be perfectly truthful and still exclude the foundation, the site work, the trenches, the hookups, the permits, and the inspections that turn a structure into a home someone can legally occupy. Utility connections sit at the top of that excluded list because they are the least visible: the money buys you pipe in the ground and a meter on the wall, not anything you can photograph for a brochure.

They are also the least negotiable. Your builder can sharpen a framing bid. Nobody sharpens the town's connection charge. The fee is whatever the fee schedule says this year, payable to the town or the utility, and in some towns it must be paid, and the service established, before a building permit application will even be accepted. That last part matters as much as the dollar figure: utilities are not just a cost, they are a sequence, and getting the order wrong stalls a project that was otherwise ready to go.

Water: one fee, one trench, one decision

A water connection has three separate pieces. The town's connection fee buys permission and a tap. The physical work, trenching from the street or the main house and running the line, is its own cost through your contractor. And the metering decision, shared with the house or separately metered, is a choice you make once, cheaply, at construction, or later, expensively, with a shovel.

Builders report town water connection fees ranging from several thousand dollars to $15,000 depending on the community, and some towns put water first in the process: service established, then permit application. If your town works that way, the water department is your first phone call, not your last.

On the metering question, a separate meter gives the ADU its own bill. That is worth little if a parent lives there forever and worth a great deal the day the unit becomes a rental, because "we split the utilities informally" is a sentence that has started a thousand family arguments and complicated more than a few tenancies.

Sewer: the fee with the widest spread in the state

If the street has municipal sewer, the ADU pays to join it, and no fee varies more from town to town. The reason is capacity. A sewer system is an engineered thing with a finite number of gallons per day it can treat, and towns ration that capacity through their connection charges and their rules.

Kingston is a useful example of how seriously towns take this. Its sewer regulations allocate capacity per property using the same design-flow math as Title 5, presume each single-family residence at three bedrooms, and require Sewer Commission approval to expand beyond that. Builders working there report the connection package running around $26,000. Whether your town's figure is a quarter of that or all of it, the structure is the same everywhere: the fee is written down, the capacity rules are written down, and both are available to you before you commit to anything.

No sewer? Then the question is Title 5

On a septic lot there is no connection fee to pay, and something harder takes its place: your system's rated capacity. Massachusetts sizes septic systems by bedroom count under Title 5, at 110 gallons per day per bedroom, so an ADU bedroom is a real load on a system that was engineered decades ago for a specific house. Sometimes the answer is a dedicated tank for the ADU, which builders commonly report in the $10,000 to $12,000 range for the tank work. Sometimes it is upsizing the main system to serve both dwellings. And on a difficult lot, poor soils, high water table, tight setbacks, the septic answer can cost more than the foundation, which is why it belongs at the front of feasibility, not the end. We wrote a full guide to that question: Title 5 First.

Electric: a load calculation, not a guess

How an ADU gets its power is an engineering question with a legal answer: a licensed electrician runs a load calculation for what the unit will actually draw, heat, hot water, cooking, laundry, and any EV charging, and the calculation decides whether a subpanel from the main house can carry it or the ADU needs its own service from the street.

Full-size units with electric heat and appliances draw enough that many builders default to a dedicated service, and there is a second argument for one beyond capacity: a separate service means a separate meter and a separate account, which does for electricity exactly what the separate water meter does for water. But this is a decision your electrician prices for your unit on your street, not a rule of thumb to budget from. What you should insist on is seeing the load calculation and the service decision in writing as part of the quote.

Shared or separate: decide it by the exit, not the entrance

Running the ADU off the main house's existing services is allowed, and it is genuinely cheaper on day one. The honest way to make the choice is to ignore day one and ask what the ADU is for on day five thousand. A unit built for family that will only ever house family loses little by sharing. A unit that might someday be rented, or sold on its own if the property is ever divided into condominiums, wants its own water, its own power, and its own bills, because retrofitting separation later means reopening finished ground and finished walls at a multiple of the original cost, and separately metered services make any future rental or condominium conversation dramatically simpler for the attorneys and lenders who will run it.

The point

Utility costs have a reputation as the ambush at the end of an ADU project. They do not deserve it. Every one of these numbers, the water fee, the sewer fee, the septic requirement, the electrical service, exists in a public document or a site evaluation that can be in your hands before you commit a dollar. A builder who prices them up front is showing you a real budget. A builder who leaves them off is showing you a headline. The difference between the two is a fee schedule and five phone calls, and now you know which calls to make.