ADU Design Cost in 2026 (Granny Flat & Backyard Home Plans)
Designing an accessory dwelling unit (ADU) — a granny flat, backyard cottage, or garage conversion — typically costs $1,200 to $5,000 for a plan set in 2026, or $2,000 to $10,000 for fully custom plans. If you hire a licensed architect for full service, budget 8% to 15% of the ADU's construction cost. Because an ADU is essentially a small home, its design follows the same pricing as house plans and additions.
How ADU design is priced
An ADU sits between a home addition and a small custom home, and its design pricing reflects that.
| Approach | Typical cost | Best for |
|---|---|---|
| Pre-drawn / stock ADU plans | $700–$1,500 | Standard units on standard lots |
| ADU plan set (drafter/designer) | $1,200–$5,000 | Most detached and converted ADUs |
| Custom ADU plans | $2,000–$10,000 | Unique designs or tricky sites |
| Full-service architect | 8%–15% of construction cost | Complex, high-end, or difficult sites |
| Garage / space conversion drawings | $1,200–$5,000 (as an addition-type set) | Converting existing structures |
On a $150,000 detached ADU built with full architectural service at 12%, design fees would be about $18,000. Most homeowners, though, use a drafter or designer and spend a few thousand on plans.
What the design package includes
A complete ADU plan set generally covers:
- Floor plans for the unit's living space, kitchen, and bath
- Elevations showing the exterior
- A foundation plan for the new footprint (or the existing one, for conversions)
- Structural details for framing, beams, and connections
- Cross-sections and enough detail to permit and bid
- A site plan ($100 to $1,500) showing where the ADU sits, setbacks, and utility connections
For a garage or basement conversion, you may also need as-built drawings ($700 to $1,300) of the existing structure so the designer knows exactly what they're working with.
Extra drawings to budget for
ADUs almost always require supporting documents beyond the main plans:
| Document | Typical cost |
|---|---|
| Site plan | $100–$1,500 |
| As-built drawings (for conversions) | $700–$1,300 |
| Altering existing / stock plans | $150–$2,500 |
| CAD drawings (per sheet) | $35–$180 |
Utility connections (water, sewer, electrical) and any required soils or structural engineering are typically separate — worth asking about early, since they can affect both the design and the permit.
What drives ADU design cost
- Detached vs. conversion. A detached unit needs a full plan set; converting a garage may lean on as-builts plus alteration drawings.
- Size and layout. More square footage and complex layouts add design hours.
- Site conditions. Slopes, setbacks, tight access, and utility runs all add work.
- Local ADU rules. Many jurisdictions have specific ADU codes and, in some places, pre-approved plan programs that can lower design cost.
- Custom vs. stock. A pre-drawn ADU plan is the cheapest start; full custom costs the most.
- Who you hire. A drafter or designer costs less than an architect for the same buildable set.
How to keep ADU design affordable
- Check for a pre-approved plan program. Many cities now offer permit-ready ADU plans that cut design cost and time.
- Start with a stock ADU plan and modify it rather than designing from scratch.
- Get accurate as-builts before converting an existing structure.
- Match the pro to the job — a straightforward backyard unit rarely needs a full-service architect.
- Compare a few vetted local designers to see who understands your city's ADU rules and offers the best value.
Do you need an architect?
For a standard detached ADU or a simple conversion, a draftsman or home designer can usually deliver a permit-ready set for $1,200 to $5,000. For a difficult lot, a two-story unit, or a high-end design, an architect's fee (8% to 15% of construction cost) often pays off in a better result and smoother permitting.
Bottom line: budget $1,200 to $5,000 for ADU plans (up to $10,000 custom), plus a site plan and possibly as-builts — and check whether your city offers pre-approved plans before you start.