The cost to build a DADU

Foundation and Hook Up Utilities

DADU Construction Costs: Foundation & Utility Hookups

2nd phase of the journey: Laying Your DADU's foundation

After pre-construction is complete — design approved, permits in hand, and the site ready to break ground — a DADU project moves into its first true construction phase: foundation work and utility connections. This is where progress becomes visible, and where it is important to keep track of costs.

Below is a clear breakdown of what to expect.

Foundation work is the first moment a DADU feels real.

Lines appear on the ground, forms go up, and suddenly months of drawings and approvals turn into something you can walk around. For our clients, this phase is both exciting and unexpectedly fascinating. It’s the point where the project shifts from concept to construction, and where the quality of the unseen work below grade determines the performance, durability, and comfort of the living space above.

This property required pin piles do to soil considitons

Rebar Installation

Common DADU Foundation Types

If you’re not deeply familiar with construction, building a DADU tends to be an education in itself. The learning curve starts early, often with the realization that not all foundations are created equal. What seems like a single step is actually a set of distinct approaches, shaped by site conditions and structural needs.

Slab-on-Grade (most common)

Excavation and grading
Rebar, vapor barrier, concrete pour
Typical Cost: $15,000 – $30,000

Crawl Space Foundation

Footings and stem walls
Additional excavation and inspections
Typical Cost: $25,000 – $45,000

Pier & Beam (sloped or constrained lots)

Drilled piers or piles
Beams and subfloor framing
Typical Cost: $30,000 – $60,000+

Utility Hookups: The Hidden Variable

Utility connections are one of the most underestimated DADU expenses. Distance, trenching, and municipal requirements can dramatically affect costs.

It’s also worth noting that foundation and utility costs often sound more intimidating than they are in practice. While site conditions can push numbers up or down, many projects land comfortably within typical ranges.

In addition, in King County, most properties that are connected to the public sewer system on or after February 1, 1990, are subject to a sewage treatment capacity charge. Rather than a single upfront fee, this charge is billed quarterly for up to 15 years (or until the balance is paid off) to help fund regional wastewater infrastructure, and it is separate from the regular sewer service bill.

Sewer

Tie-in to existing house lateral or city main
Typical Cost: $5,000 – $25,000+

Water

- - Branch line from existing service
  - Some jurisdictions require a separate meter
    Typical Cost: $3,000 – $10,000
    With new meter: +$5,000 – $15,000

Electrical

- - Subpanel from main house or full service upgrade
    Typical Cost:
  - Subpanel: $4,000 – $8,000
  - Service upgrade / new meter: $8,000 – $20,000

Phase	Rough Range
Foundation	$15K - $35K
Utilities	$15 - 35K
Subtotal	$30k -$70K

What Comes After the Foundation

Once the foundation is poured and utilities are in place, the project moves into framing and dry-in — walls go up, the roof goes on, and the DADU finally starts to look like a building. This is often the biggest single cost jump in the entire project and the point where construction momentum accelerates.

Keep in mind, plumbing and electrical costs are not incurred all at once. Each typically spans three phases over the course of construction—hook-up, rough-in, and final connections—with costs distributed accordingly as the project progresses.