ADU & Guest House Concrete Slabs: Foundation Basics for Prescott
Concrete slab foundations for ADUs and guest houses in Prescott, AZ — post-tension vs conventional, thickness, insulation, and how permits differ from a house.
ADU (accessory dwelling unit) construction in Prescott, Prescott Valley, and Chino Valley has picked up significantly since local ordinances loosened. The slab is roughly 8–12% of total ADU cost and it's what everything else sits on for the next 50 years.
Slab options for ADUs
Monolithic slab on grade (most common)
Turned-down edges at 12–18 inches deep, 4–6 inch interior slab, all poured in one operation. Fast, cost-effective, and standard for single-story ADUs under 1,200 sq ft.
Stem wall foundation
Footings and stem walls poured first, slab poured on top of a compacted fill inside the walls. Used when the site is sloped or when a crawlspace is preferred for utilities.
Post-tension slab
Reinforced with tensioned steel cables. Overkill for most ADUs but sometimes required by soils reports on expansive clay in parts of Prescott Valley and Chino Valley.
What Prescott building code requires
- 3,000 PSI minimum on residential slabs, we spec 4,000 for durability
- R-10 rigid insulation under the slab in most residential ADU applications
- Vapor barrier (6-mil poly minimum) under the slab and insulation
- Reinforcement per soils report — often #4 rebar 16-inch centers each way
- Frost-depth footings at 18 inches around the perimeter
Utility rough-in during the slab pour
Everything below-grade goes in before the pour: plumbing supply and drain lines, radon vent if soils require, electrical conduit runs to floor outlets, HVAC ductwork if in-slab, radiant heat tubing if applicable. Cutting into a finished ADU slab is exponentially more expensive than doing it right the first time.
What a slab costs on a typical Prescott ADU
For an 800 sq ft ADU with a monolithic slab, expect $10,000–$15,000 for the slab alone including base, insulation, vapor barrier, rebar, and utility rough-in coordination. Stem-wall foundations run 20–35% more.
How permits work
ADU foundations are permitted separately from the ADU itself in most Prescott-area jurisdictions. Foundation inspection happens before you close the walls. Our scope always includes pulling the foundation permit and calling the inspections.
If you're planning an ADU or casita in the Prescott area, bring us into the conversation early — the slab dictates almost everything about how the rest of the build goes.
Family-owned, woman-owned Arizona contractor (ROC #366257) with 20+ years of combined concrete, solar, and remodeling experience serving the Prescott area. Meet the team →