What an easy lot looks like
Our published prices assume a level, clear building area and a short tie-in — roughly 20 feet — to your existing well and septic. That describes a lot of New Hampshire backyards: a flat spot near the house, no rock at the surface, machines can drive right to it. On a lot like that, the number in the cost guide is the number. Everything below is what happens when a lot departs from that baseline — and how far it departs is what your quote prices.
Ledge
Ledge is bedrock close to the surface, and New Hampshire has plenty of it. A foundation and a utility trench both need to get below grade, and when the excavator hits rock instead of soil, the digging turns into hammering or blasting. Ledge removal is priced work — it shows up as a line item, sized to how much rock is in the way and where. It rarely stops a project, but it's the single most common reason two neighboring lots quote differently, and it's exactly the kind of thing we want found during assessment, not during excavation.
Slope and grading
Our ADUs sit on an insulated monolithic slab, and a slab wants level ground. A gentle grade gets cut and filled as part of normal site prep. A real slope means more earth moved, possibly retaining, and more thought about where water goes when it rains. Slope also interacts with the other checks — sometimes the flattest spot on the lot isn't inside the setbacks, and the buildable spot needs grading. The visualizer shows you where a footprint legally fits; the site walk tells us what that spot costs to prepare.
Septic capacity — and the redesign question
An ADU adds bedrooms, and in New Hampshire septic systems are sized by bedroom count. If your existing system has spare capacity, the ADU ties in and life is simple. If it doesn't, the system needs to be expanded or redesigned — and a septic redesign is one of the major site surprises that sits outside the all-in price, alongside ledge removal. This is a records-and-fieldwork question: what the system was designed for, what it serves now, and what the soil allows. It's answerable before you commit to anything, which is exactly when you want it answered.
Trees, clearing, and truck access
A wooded building spot has to be cleared and stumped before anyone can dig, so tree work is a real line on a wooded lot and a zero on an open one. Access matters just as much: concrete trucks, delivery trucks, and an excavator all have to reach the building area. A wide driveway with a straight shot to the backyard is easy. A tight side yard between the house and the property line — remember, the ADU itself needs 10 feet of separation from existing structures — can force smaller equipment and more handling, which is time, which is money. This is one of the first things we look at on a site walk.
The utility run — and how we find all this before you sign
Every foot of trench between the ADU and its water, sewer or septic, and power tie-ins is dug, laid, and backfilled. A unit placed near the existing utilities keeps that run short; a placement at the far corner of the lot pays for the distance — one reason placement is a cost decision, not just a looks decision. If you're connecting to town water or sewer, the town's tap fees apply too; the town sets those, not us. Our process finds all of this in order: the free zoning review confirms what the law and your town allow, the $250 on-site assessment (credited toward your build) walks the lot for utilities, septic, ledge, grade, and exact placement — and then every site condition we found is written into the quote as a named line item before you sign. Compare the models once you know what your lot wants to carry.