The most common question we hear from San Antonio homeowners is also the most honest one: "What's this actually going to cost me?"
The answer depends on your lot, your goals, and the type of ADU you build — but it's not a mystery. Here's how to think through the numbers before you ever talk to a builder.
The short answer: $160,000–$320,000 in San Antonio
A detached ADU in San Antonio — designed, permitted, and built to a high standard — typically runs between $160,000 and $320,000 all in. That range is wide because ADUs vary significantly in size, complexity, and finish level. A 400 sq ft garage conversion sits at the low end. A 750 sq ft new-construction casita with a full kitchen, bathroom, and high-performance envelope sits at the upper end.
Garage conversions and attached ADUs generally cost less because they reuse an existing structure and don't require a full foundation. Detached new construction costs more because you're building a complete standalone home.
Where the money actually goes
Most homeowners think of ADU cost as a single number. In practice, it's five separate cost buckets — each driven by different variables.
| Cost category | Typical range | % of total |
|---|---|---|
| Design & architecture | $8,000 – $18,000 | 5–8% |
| Permits & city fees | $3,000 – $10,000 | 2–5% |
| Site work (foundation, utilities) | $15,000 – $40,000 | 8–15% |
| Construction (shell + interior) | $110,000 – $220,000 | 65–75% |
| Soft costs (surveys, insurance, PM) | $5,000 – $15,000 | 3–6% |
| Total (detached, 500–750 sq ft) | $160,000 – $320,000 | 100% |
The five variables that move your number
1. ADU type and size
The biggest lever. A 400 sq ft studio costs less than a 750 sq ft one-bedroom — both in materials and in permit/impact fees, which in San Antonio are calculated partly on square footage. Garage conversions save on foundation and framing costs but often require more electrical and plumbing work than people expect.
2. Site conditions
This is the one that surprises people most. If your lot has challenging grade, poor soil bearing capacity, limited utility access, or a tight setback situation, site prep costs climb fast. That's why a proper site assessment — before you commit to a design — is money well spent. You want to know about a $25,000 drainage problem before you're already mid-design.
3. Design path: permit-ready vs. custom
A permit-ready plan from a pre-approved library costs $8,000–$12,000 in design fees and moves through San Antonio DSD faster. A fully custom design runs $14,000–$18,000 and takes longer to permit. For most homeowners, permit-ready is the smarter choice — you get a tested design, a faster timeline, and lower design fees.
4. Finish level
Builder-grade finishes (durable, clean, tenant-friendly) vs. high-end finishes (custom tile, designer fixtures, premium appliances) can swing total cost by $20,000–$40,000 on a 600 sq ft unit. For a rental-focused ADU, builder-grade is almost always the right call. For a family member or personal use, you may want to invest more.
5. Performance specification
At Casita Fresca, every unit is designed to Passive House principles — better insulation, tighter air sealing, and mechanical ventilation. This adds roughly $8,000–$15,000 upfront but reduces tenant utility costs by 40–60% and significantly reduces vacancy risk. In San Antonio summers, a casita that stays cool without running the AC all day is a real competitive advantage in the rental market.
Want a real number for your specific lot?
Our $525 site assessment gives you a preliminary project budget, zoning confirmation, and a financing pre-qualification review — applied to your build cost if you move forward.
Reserve your site assessment →San Antonio permit costs: what to expect
San Antonio Development Services Department (DSD) fees for ADUs depend on project valuation and square footage. In practice, most homeowners pay $3,000–$8,000 in combined permit, plan review, and inspection fees for a standard detached ADU.
San Antonio does not currently charge impact fees on ADUs under 750 sq ft, which saves San Antonio homeowners $5,000–$20,000 compared to some other Texas cities. This is a meaningful advantage — take it seriously when comparing total project costs.
Permitting in San Antonio typically takes 6–12 weeks through DSD's standard review process. Working with a builder who has an existing relationship with DSD and uses pre-approved plans can reduce this to 4–8 weeks.
What about financing?
Most San Antonio homeowners finance their ADU through one of three paths:
- Cash-out refinance — if you have significant equity and current rates work for you
- HELOC or home equity loan — draw on existing equity without refinancing your primary mortgage
- ADU construction loan — a dedicated construction product, typically converting to a standard mortgage at project completion
As part of our program, we connect you with lenders who specialize in ADU construction financing. We handle the introduction — you make the final call on what works for your situation.
Is it worth it?
For most San Antonio homeowners: yes, with the right project. San Antonio rental demand is strong, particularly for smaller well-designed units in established neighborhoods. A 600 sq ft, high-performance ADU in the right zip code can generate $1,400–$2,200/month in rental income — enough to cover a financing payment and start building equity in the unit itself.
Property value typically increases by $120,000–$200,000 when a permitted ADU is added, depending on the neighborhood and unit quality. At a $200,000 build cost, that's a strong equity position from day one — before a single rent check arrives.
The caveat: a poorly designed or poorly built ADU can underperform on all of these metrics. Vacancy, maintenance costs, and tenant turnover erode returns fast. A casita that's comfortable, well-built, and properly permitted is the investment. Everything else is just a box.
Bottom line
In San Antonio in 2026, a well-designed, fully permitted detached ADU costs $160,000–$320,000 depending on size, site, and specification. That number gets real when you do a site assessment — which is exactly where we start.
If you're ready to find out what your specific lot and budget look like, reserve a site assessment. If you'd rather talk it through first, email David directly at dkomet@urbanearth.us.