ADU Groundwork: The Site Prep Nobody Budgets For

Most ADU budgets are built around the structure. The unit, the finishes, the appliances. The part that regularly gets underestimated is everything that happens before the structure exists and everything needed to put the property back together afterwards.

Site work sets the schedule for the whole build

On an ADU project the ground work happens first and gets inspected first, and everything downstream waits on it. A failed compaction test or a footing that has to be re-excavated does not delay one trade, it pushes every subsequent trade back.

That is worth understanding when you are comparing timelines. A schedule that looks aggressive at the ground stage is usually a schedule that will slip.

What clearing and grading actually involves

Removing what is there, which frequently includes more than the visible: old slabs, footings from a previous structure, buried debris, tree roots, sometimes an irrigation system nobody documented.

Then cut and fill to the plan elevations, then compaction, which gets tested and inspected. Fill that is placed without proper compaction settles later, and settlement under a structure is not something you correct afterwards.

Access matters more than people expect and is worth checking early. A rear-yard ADU on a lot with a narrow side yard may not have a route for the equipment the job needs, and the answer sometimes involves removing and rebuilding a fence or a section of wall.

Utilities: trench once

Water, sewer, gas, electrical and any low-voltage all have to reach the unit. Planning them as one coordinated trenching exercise rather than four separate ones saves real money.

Opening the same ground repeatedly costs labour every time. It also risks routes conflicting with footing locations, and it destroys whatever landscape went back in between. We plan the routes against the footing plan before anything is dug.

Sewer is the one that most often produces a surprise. Where the connection has to be made, and at what depth, determines whether the run is straightforward or whether it needs to cross the property at a depth that affects everything else in the trench.

Drainage changes the moment you add a building

A new roof sheds water somewhere. A new pad interrupts a path water previously took across the lot. Both change how the property drains, and the time to deal with that is while the ground is already open.

Finishing an ADU and then discovering the back corner of the garden now floods is common and entirely avoidable. It is also much more expensive to solve after the unit is finished, because the easy trench routes are now under something.

The part that gets left out of most site packages

When the structure is finished, the property around it looks like a construction site, because it was one. Bare compacted ground, no path to the door, damaged planting, and a drainage pattern that has changed.

Walkways, flatwork, drainage, restoration planting and lighting are what make the finished unit read as part of the property rather than a box that landed in the back garden. This is the stage most site budgets omit, and it is the part the owner and the tenant actually look at every day.

It is also the stage where the ADU stops feeling like an addition and starts feeling like it belongs, which matters if you are renting it or housing family in it.

Parking and the driveway

Frequently part of the same scope. Additional parking, a widened approach or a new pad often accompany an ADU, and doing that work while the equipment is already on site is considerably cheaper than bringing it back.

Who does what

Our scope is the ground: clearing, grading, compaction, footings and pads, utility trenching, retaining where the grade requires it, drainage, flatwork and the landscape restoration at the end. We work to the stamped plans and coordinate with the designer, the engineer and whoever is building the structure.

Structural permits normally sit with the designer and the general contractor. We handle the permits that apply to our scope and work to the overall inspection schedule.

If you are planning one

Get the site looked at early, before the design is finalised. Access, drainage, where utilities can run and what is buried in the yard all influence what is sensible to build and where to put it. Finding that out during design is free. Finding it out during construction is a change order.

What turns up in the ground

On older properties, particularly through the post-war tracts across the north of the county, excavation regularly finds things nobody documented.

• Old foundations or slabs from a previous structure, sometimes a garage or shed.
• Buried construction debris used as fill during the original build.
• Abandoned utility runs, irrigation mains and occasionally a septic system that predates the sewer connection.
• Tree roots from specimens removed decades ago.
• Fill of unknown compaction where the lot was originally graded.

None of these are disasters, but they cost time and money and they change the plan. When we hit something, we stop, document it, tell you what it means for cost and schedule, and get a decision. What we will not do is absorb it quietly and produce it as a surprise on the final invoice.

Setbacks, access and where the unit can actually go

Your designer sets the position against the code and the lot. What we look at is whether that position can physically be built, which is a different question.

A rear-yard unit on a lot with a four-foot side yard may have no route for a mini excavator or a concrete truck’s chute. The answers are usually removing a fence section, pumping concrete rather than chuting it, or in some cases accessing from a neighbouring property with permission. All of those are solvable and all of them cost something, so finding out during design is far better than finding out on the first morning.

What the inspection sequence looks like

Ground work is inspected in stages, and each one is a gate the next stage waits behind.

• Compaction of the pad, often with testing.
• Footing excavation, before any concrete.
• Reinforcement placement, before the pour.
• Underground utilities, before backfill.

Each of these means scheduling an inspector and working to their availability. That is the main reason permitted work runs longer than unpermitted work, and it is not compressible by pushing the crew.

The landscape budget that gets cut

By the time the structure is done, budgets are tight and the yard is a mess. This is exactly when the restoration gets deferred, and it is the wrong thing to defer.

A finished unit with no path to its door, bare compacted ground around it and a drainage pattern nobody addressed is a unit that looks temporary and drains badly. If you are renting it, that shows in what you can charge. If family is living in it, it shows every day.

Our advice is to carry the restoration in the main budget from the start rather than treating it as a phase two that may not happen.

Common questions