ADU Site Preparation in Portland: Excavation, Utilities & What It Costs

Portland has been one of the most ADU-friendly cities in the country since it relaxed accessory dwelling unit regulations. Detached ADUs — small standalone homes built in the backyard — are going up across the metro area, from inner SE Portland to Beaverton to Happy Valley. They're used as rental income, multigenerational housing, home offices, and guest houses.

But before a builder can frame a single wall, the site needs to be prepared. That means clearing, grading, digging utilities, and excavating the foundation — all in a backyard that usually has limited access, existing landscaping, and tight setbacks. Here's what's involved.

What Makes ADU Site Prep Different

An ADU in a backyard is not the same job as a new home on an open lot. The differences matter because they affect cost and complexity.

Access. Equipment needs to get to the backyard. If there's a side yard wide enough for a mini-excavator (about 5 feet), that's the path. If not, sections of fence come down, or equipment goes through the house's garage. Some ADUs in tight inner-Portland lots require hand digging for portions of the work. Limited access means smaller equipment, more time, and higher cost.

Proximity to the main house. The ADU foundation is typically 15 to 30 feet from the existing house. Excavation vibration and trenching near the main house's foundation need to be managed carefully. Utility trenches often run parallel to or across the main house's existing underground services — accurate utility locates are critical.

Existing landscaping and hardscape. The ADU footprint usually takes out part of the existing yard — lawn, garden beds, trees, patios, or fences. Some of this can be preserved, some can't. The scope of what gets disturbed and what gets restored after construction affects cost.

Setbacks. Portland's ADU setback requirements determine exactly where on the lot the structure can go. The site prep crew works within those constraints, and the building pad needs to be precisely placed.

The Site Prep Process

1. Clearing

Clearing the ADU footprint and access route. This means removing any trees (with permits if they're over 12 inches in diameter), shrubs, garden beds, lawn, fences, sheds, or hardscape in the way. The cleared area needs to be larger than the ADU footprint — the crew needs working room around the foundation for excavation and forming.

Typical cost: $1,000 to $5,000 depending on what's in the way. An open lawn area with no trees is cheap. A heavily landscaped backyard with a large tree, a shed, and a patio is more.

2. Rough Grading

Grading the building pad area to the correct elevation. The ADU foundation plans specify the finished floor elevation relative to the existing ground. The crew grades the pad area level at the right height, with drainage sloping away from both the ADU and the main house.

Typical cost: $1,500 to $4,000. Flat backyards are straightforward. Sloped backyards — common in Portland's West Hills, Sylvan, and Marquam neighborhoods — may need retaining walls or significant cut-and-fill work, pushing costs higher.

3. Utility Trenching

The ADU needs its own connections for water, sewer, electrical, and possibly gas. Utility trenches run from the ADU location to either the main house's connections (if the city allows shared services) or to independent connections at the street.

Water: A new water line trenched from the ADU to the main water service or a separate meter. Trench depth: 30-36 inches minimum in Portland.

Sewer: A sewer lateral from the ADU to the main sewer line or a new connection to the city sewer in the street. Sewer lines need proper gravity slope — 1/4 inch per foot minimum. The depth depends on the connection point depth.

Electrical: Conduit trenched from the main house's panel (if the panel can handle the additional load) or from a new service at the street. Trench depth: 18-24 inches.

Gas: If the ADU has gas appliances, a gas line trenched from the main house's gas meter or a new meter.

Typical cost for utility trenching: $3,000 to $8,000 for the excavation work. The actual pipe and wire installation is done by licensed plumbers and electricians — their costs are separate. City connection fees (SDCs — System Development Charges) are also separate and can run $5,000 to $15,000+ in Portland, though the city has offered SDC waivers for ADUs in some years.

4. Foundation Excavation

Foundation excavation for an ADU is similar to a small house but in a tighter space. Most Portland ADUs use either a slab-on-grade or a crawlspace foundation. The crew excavates to the depth and dimensions specified in the structural plans, prepares the base with compacted gravel, and leaves it ready for the foundation contractor.

Typical cost: $3,000 to $8,000 depending on foundation type, soil conditions, and access. A slab-on-grade on a flat lot with good access is on the lower end. A crawlspace foundation on a sloped lot with tight access is higher.

5. Stormwater

Portland requires stormwater management for new impervious surfaces. The ADU's roof area is new impervious surface that needs to be managed — typically with a dry well, soakage trench, or rain garden. The excavation for the stormwater system is usually done during the utility trenching phase.

Typical cost: $1,500 to $4,000 for the stormwater system excavation and installation.

6. Erosion Control

Silt fencing, straw wattles, and a stabilized construction entrance are required during construction. Portland enforces this — inspectors check sites and issue fines for violations.

Typical cost: $500 to $1,500 for materials and installation.

Total Site Prep Budget for a Portland ADU

For a typical detached ADU on a standard residential lot in the Portland metro area, budget roughly $10,000 to $30,000 for the complete site preparation scope.

Lower end ($10,000 to $15,000): flat backyard, easy access, minimal clearing, simple slab foundation, short utility runs.

Middle range ($15,000 to $22,000): moderate clearing, some slope, crawlspace foundation, 50-80 foot utility runs, stormwater system.

Higher end ($22,000 to $30,000+): significant slope, limited access (mini-excavator or hand work), tree removal, retaining walls, long utility runs, complex soil conditions.

These numbers cover the excavation and site work only. They don't include the SDC fees, the building permit, or the actual construction of the ADU.

Common Mistakes in ADU Site Prep

Not checking access before design. If the only path to the backyard is a 4-foot-wide gate, that limits equipment options and drives up cost. Walk the access route with your excavation contractor before finalizing the ADU design — sometimes shifting the ADU location by 10 feet opens up a much better equipment path.

Ignoring the main house's foundation. Excavating close to the existing house requires care. If the ADU foundation is deeper than the main house's footings and within a 45-degree influence zone, underpinning or stepped footings may be required. The geotech report should address this.

Underestimating utility costs. The trenching is one cost. The plumber, electrician, and city connection fees are separate — and often larger. Get all the numbers before committing to the project budget.

Forgetting surface restoration. After construction, the yard around the ADU looks like a construction site. Restoration — regrading, topsoil, seeding or sod, fence repair, pathway installation — is a real cost that's easy to forget in the budget.

Get Started

If you're planning an ADU in the Portland metro area, call (503) 983-6984 or request a free estimate. We handle the full site prep scope — clearing, grading, utility trenching, foundation excavation, and stormwater — so your builder has a ready-to-go site.