Garage-to-ADU Conversion: Demolition and Site Prep (Oregon)

Quick Verdict

Garage to ADU demolition prep in Oregon usually means deciding whether the existing slab and footings can stay, trenching new utility lines to the unit, and grading the pad for a foundation that meets code for living space. Most garages were never built to hold a habitable structure, so the footings often need to go deeper, which means breaking out and removing part or all of the slab. Add utility trenches for water, sewer, and power, plus erosion control and a clean finish grade, and you have a real earthwork scope before a single wall goes up. This page covers the demo-and-site-prep portion only, not the build. Oregon's ADU-friendly rules make the project worth it, but soil and permits drive the budget.

What an ADU Conversion Demands Before the Build

Converting a garage into an accessory dwelling unit changes how the structure has to perform. A garage carries a vehicle on a thin slab; an ADU carries people, plumbing, insulation, and a building inspector's expectations. That shift in use is what triggers the demolition and excavation work.

The site-prep scope typically includes some mix of these tasks:

• Removing all or part of the existing slab so footings can be deepened or replaced
• Trenching for new water, sewer or septic, electrical, and sometimes gas
• Excavating and forming new footings that meet frost depth and bearing requirements
• Rough and finish grading for positive drainage away from the new walls
• Erosion and sediment control during the disturbance window

For a deeper look at how demolition is sequenced and handled, see our residential demolition guide.

Does the Slab Stay or Go?

This is the first real decision. If the existing footings are deep enough and the slab is sound, you may be able to keep it and build on top. More often, an ADU foundation needs footings below the garage's shallow turn-down edge, and the only way to reach that depth is to saw-cut and break out the perimeter or the whole slab.

Pulling the slab also lets you inspect what is underneath. Old garage slabs were frequently poured over uncompacted fill, organic soil, or buried debris. If the subgrade is soft, you do not want to find out after framing.

Current Market Reality

Slab removal and haul-off is its own line item. When you hit reinforced concrete, thick pours, or an unexpected second slab, costs climb fast. For how stand-alone garage teardowns are priced, our detached garage demolition cost breakdown is a useful reference even when you are converting rather than demolishing.

Utility Trenching for the New Unit

A garage rarely has the plumbing an ADU needs. That means trenching from the house or the main connections to the new unit for water and sewer, plus pulling electrical and possibly gas. In Oregon, utility trenches must hit minimum cover depths and clear other lines, and the work follows county permit steps and inspections.

Before any digging, the site gets an 811 locate so existing buried utilities are marked. Private lines, like a septic lateral or a sprinkler main, are not covered by 811 and must be located separately.

Trench run	Typical depth driver	Notes
Water service	Below frost, protected	Deeper east of the Cascades
Sewer or septic lateral	Gravity fall to main or tank	Grade matters more than depth
Electrical	Code cover for conduit type	Often shares a common trench

A clean, well-planned trench layout keeps inspections smooth and avoids re-digging.

Footing Depth, Clay, and Rock

Oregon soil decides how hard the foundation excavation is. In the Willamette Valley, saturated clay can be soft at footing depth and may require over-excavation and structural fill. In Central Oregon, basalt and rock can stop a bucket cold, forcing ripping or hammering to reach the required depth. Coastal sites bring sand and high water tables.

The footing has to be fully embedded into firm bearing soil at the right depth, which is why a garage's shallow edge usually is not enough. If the inspector or engineer flags soft material, expect an undercut-and-replace step.

Grading the Pad and Controlling Water

Once the demo and trenching are done, the pad gets graded so water drains away from the new walls on all sides. Positive fall, compacted fill in lifts, and a clean finish grade are what protect the ADU from the ponding and moisture that ruin foundations in wet Oregon winters. Erosion control stays in place until the ground is stabilized.

What This Phase Costs

Pricing the demo-and-site-prep scope depends entirely on the slab decision, trench lengths, soil, and haul-off. Here are planning ranges for the earthwork portion, not the ADU build.

Unit	Baseline Range
Excavator + operator, hourly	$150 - $350+ per hour
Dump truck haul-off, per load (10-14 cu yd)	$250 - $750+ per load
Trenching, per linear foot	$8 - $40+ per linear foot
Grading / leveling, per sq ft	$0.75 - $4.00+ per sq ft
Residential permit pull	$100 - $600+ (varies by jurisdiction)
Mobilization fee	$250 - $800+ flat

Industry Baseline Range: a typical garage-to-ADU site-prep scope can run anywhere from a few thousand dollars for a simple keep-the-slab job to well over $20,000+ when full slab removal, deep utility runs, and soft-soil undercut all stack up. These are industry baseline ranges for planning only -- actual pricing depends on site conditions, soil, access, depth, haul-off, and current market conditions. Get a site-specific quote.

The Bottom Line

A garage-to-ADU conversion is a smart use of Oregon's friendly accessory-dwelling rules, but the demolition and site prep is real construction work, not a weekend cleanout. Get the slab decision, utility trenches, and foundation grade right and the rest of the build goes smoothly. Cojo is CCB licensed and insured and handles the demo, trenching, and grading side of these conversions across Oregon. Learn more about our excavation services or request a free estimate for your conversion. For smaller interior tear-out, our interior selective demolition guide covers that scope, and the Oregon excavation contractor guide ties the whole process together.