Site preparation can move a backyard cottage budget more than the building itself. The short answer is that DADU site work costs depend on slope, soil, access, trees, drainage and anything already standing in the way. A DADU, or detached accessory dwelling unit, is a separate home on the same lot as a house. Two identical cottages can need very different amounts of digging, hauling and protection before framing starts, which is why the lot deserves study first.

Details below reflect official Seattle pages checked in October 2026. Confirm with city staff for your property.

What Site Work Usually Covers

Site work is everything that prepares the ground for a foundation and restores the yard afterward. Typical tasks include:

• Demolition of sheds, patios or old garages in the building area
• Clearing vegetation and stripping topsoil
• Excavation and grading for the foundation
• Hauling soil off site or bringing fill in
• Drainage measures and erosion control
• Trenching for utility lines
• Final grading and basic landscape restoration

Each item can range from minor to major. Clear estimates list them separately, which makes it easier to see where the money goes.

Slope and Soil Conditions

A flat lot with firm soil is the simplest starting point. Sloped yards may need retaining walls, stepped footings or more excavation to create a level pad.

Soil matters just as much. Soft, wet or fill soils may call for a geotechnical report and a heavier foundation design. Seattle has many older lots with uneven fill, and nobody can confirm conditions without testing. Engineering adds a line to the budget up front, though it can prevent larger changes later.

Access for Crews and Equipment

Getting machines and materials into a backyard is a real cost driver. An alley or wide side yard lets a small excavator and delivery trucks reach the work area.

A narrow path between houses changes the picture. Crews may need smaller machines, hand digging, conveyors or extra trips with wheelbarrows. Soil export takes longer as well. Access limits rarely stop a project, but they often stretch the schedule and labor hours.

Trees and Seattle Tree Rules

Mature trees add value to a yard and can complicate construction. Roots extend well beyond the canopy, and digging near them may require protection fencing, hand work or a revised building footprint.

Seattle regulates trees under SMC Chapter 25.11, and SDCI explains the current requirements on its trees and codes page. An arborist report may be part of permit review when significant trees stand near the work area. Rules were updated in recent years, so confirm what applies to your lot early.

Drainage and Environmentally Critical Areas

Water has to go somewhere once a new roof and foundation replace lawn. Stormwater requirements can add downspout connections, dispersion trenches or other drainage features to the scope.

Some parcels fall within environmentally critical areas regulated under SMC Chapter 25.09, such as steep slopes or landslide prone zones. These lots may face extra studies and review. The city GIS maps and the ADUniverse feasibility tool can show whether a parcel carries one of these designations.

A Hypothetical Pair of Lots

Consider two example lots, invented for illustration. The first is flat, has alley access and only a small shed to remove. A compact excavator can drive in, dig footings and load trucks from the alley.

The second lot slopes toward the rear with a large tree near the planned footprint and only a narrow side path. Its owner may need a geotechnical report, a short retaining wall, root protection and hand excavation near the tree. Both lots can host the same cottage, yet the second scope is clearly larger.

Common Surprises Underground

• Buried concrete, old foundations or debris from earlier owners.
• Abandoned heating oil tanks that require decommissioning.
• Groundwater appearing higher than expected after rain.
• Fill soil that needs removal or deeper footings.
• Utility lines located in a different spot than records show.

A reserve for unknowns helps absorb these finds. The article on planning a DADU contingency walks through how that reserve works.

Frequently Asked Questions

Do I need a geotechnical report for a backyard cottage?

Not on every lot. Reports are more likely on slopes, wet ground, fill soils or parcels within environmentally critical areas. SDCI reviewers may ask for one during permit review if site conditions suggest risk. Talking with a designer or engineer early about your specific parcel is the clearest way to learn whether testing makes sense before final plans.

Can a tree stop a DADU project?

A protected tree rarely ends a project outright, though it can shape where the cottage goes and how crews dig. Seattle tree rules under SMC 25.11 may require protection measures or limit removal. Some owners shift the footprint or adjust the foundation design. Confirm current tree requirements with SDCI before you finalize a site plan.

Who handles erosion control during construction?

The builder usually installs and maintains erosion and sediment controls as part of site work, and inspectors may check them. Confirm in writing that the estimate includes these measures. Silt fencing, covered soil piles and protected storm drains are typical examples, and leaving them out of a bid can create a gap.

Is soil export a big cost?

It can be on some lots. Every truckload of excavated soil needs loading, hauling and a disposal site, and tight access slows each trip. Foundation type, slope and basement plans all change the volume. Ask builders to describe how much export they expect and whether trucking and dump fees sit inside the bid or outside it.

Pre Design Site Checklist

• Sketch slope, access paths, trees and existing structures.
• Check the parcel on city GIS maps for critical area flags.
• Ask neighbors or past owners about buried tanks or old foundations.
• Note where water collects after heavy rain.
• Locate the main house sewer and power entry points.

Understanding DADU site work costs early gives every later decision a firmer base. The overview of why Seattle DADU budgets vary places site work alongside other factors, and the post on utility connection expenses covers trenching in more depth. More topics live in the costs and budgeting section.