Case Study: The ADU Above a Garage

1. The property. A 6,800 SF corner lot; 1960s house, detached two-car garage off the side street, no rear-yard room for a detached unit the owners would accept (the yard was the kids' domain, and staying that way).

2. The goal. A 1-bedroom rental above the garage: keep the parking, keep the yard, gain the income (3.4's exact promise).

3. The constraints. The garage itself. The structural assessment (11.3 — by now you know this move) found what 3.4 warned about: unreinforced walls on a marginal slab, nowhere near second-story capacity. Also: a corner lot's two front setbacks, and a 22-foot local height allowance that made the two-level massing legal — barely.

4. Options considered. (a) Retrofit the existing garage to carry a second story — the engineer's estimate for foundation, reframing, and shear work approached the cost of… (b) demolish and build a new two-story garage-with-ADU-above — better building, cleaner engineering, nearly the same money (3.4's punchline, arriving on schedule). (c) Abandon the vertical idea. The numbers made it easy: (b), new construction wearing the old garage's footprint logic.

5. The final design. Ground floor: two-car garage plus the utility room (HPWH, panel — 7.8/7.3 equipment finding a proper home). Above: a 640 SF 1BR following 4.8's classic — living across the street-facing front with the corner-lot light, bedroom and bath at the quiet rear. Exterior stair on the inside face, landing shielded from the neighbor's yard (8.5). The corner-lot geometry meant the unit reads as a carriage house from the street — a genuine curb-appeal addition.

6. City issues. The corner lot's second front setback generated one planning round (front setbacks are the local-rule zone — 2.2); the design had anticipated it, and the 800 SF front-setback protection (5.2) settled the discussion.

7. Permitting. Two rounds, 14 weeks — the demolition permit and new-construction package running together.

8. Construction. 8 months. Demolition took four days and removed every retrofit unknown with it (the quiet advantage of option b: no surprises hiding in kept walls). The new foundation, engineered for the full two-story load from day one, poured without drama. Utilities trenched once, sized for the whole building.

9. The final result. A code-clean two-story building: cars below, a bright carriage-house 1BR above, yard untouched — the kids' trampoline never moved. Rented to a graduate student who walks to the nearby campus.

10. Cost range. $340,000–$365,000 all-in — detached-new-construction money (3.4 told you), buying a garage and a dwelling.

11. Timeline. About 15 months, assessment to lease.

12. Lessons learned. The assessment didn't kill the dream — it redirected it to the version that could actually be built well. "Keep the existing structure" is a means, not a goal; when retrofit costs approach new-construction costs, new construction wins on every axis. And above-garage units price like what they are — two buildings stacked — so let the yard-preservation and parking benefits justify the premium, or choose another path.