ADU Utility Connections: The Costs Nobody Mentions

Everyone shopping for an ADU compares the visible things: square footage, finishes, one bedroom or two. Almost nobody asks the question that quietly shapes the budget more than any of those — how do water, sewer, and power get from the street (or the main house) to a new building at the back of the lot?

It's not a glamorous topic. But in our experience designing ADUs across San Diego, utilities are the single most common source of surprise between a homeowner's first sketch and a real construction number.

Sewer: gravity is not negotiable

Wastewater flows downhill, and your ADU's drain line needs continuous fall from the unit to the point where it ties into the sewer. On a flat lot in Clairemont, that's usually straightforward trenching. On a sloped canyon lot — and San Diego has thousands — the ADU may sit below the tie-in point, which means a sewage ejector pump, a piece of equipment that adds cost up front and maintenance forever. A quick site assessment tells you which situation you have before you fall in love with a floor plan.

Power: will your panel carry a second home?

An ADU is a small house, and it wants a small house's worth of electricity. Many older San Diego homes still run on modest electrical panels that were generous in 1962 and are maxed out today. Sometimes there's room to subfeed the ADU from the existing panel; sometimes the main panel needs an upgrade; occasionally the utility needs to bring more capacity to the property. If you're also eyeing solar or an EV charger down the road, plan the capacity once — we wrote about that mindset in Solar-Ready Remodels.

Water, gas, and the all-electric question

Water supply is usually the easiest of the group, though a long run from the meter may mean upsizing the line so the main house doesn't lose pressure every time the ADU runs a shower. Gas is where we increasingly ask a different question: do you want it at all? An all-electric ADU — heat pump for heating and cooling, electric cooking and water heating — skips the gas trench entirely, simplifies the permit, and performs beautifully in our mild coastal climate. For many detached units, it's the cleaner answer.

Budget for the dirt, not just the building

The pattern behind all of this: ADU surprises live underground. Trenching across a finished yard, cutting and patching a driveway, working around mature trees or an old pool — the routing matters as much as the distance. That's why the utility plan should be part of the design conversation, not a discovery during construction, and it's why a per-square-foot number from a brochure can't be compared to a real site-specific estimate.

None of this is a reason to skip the project. ADUs remain one of the best moves a San Diego homeowner can make — we ran the rental numbers in Can an ADU Pay for Itself? — but the good outcomes start with an honest look at the ground the unit will sit on.

Thinking about a backyard unit? Start the conversation or give us a call — we'll walk your lot and tell you what the dirt will and won't allow before you spend a dollar on design.