Two provisional budgets for the same Sunnyvale ADU land side by side. The garage conversion looks less expensive because it reuses a building, but its number assumes the slab is sound—and leaves out what happens to the bicycles, tools, and car inside. The detached budget includes a new foundation and structure, yet keeps the garage in service. With the living area, layout needs, and finishes held constant, neither route is automatically cheaper.
Compare the same home, not two different projects
Start with one proposed unit: the same usable living area, bathroom and kitchen needs, level of finishes, and intended use. If the garage cannot accommodate that layout without an addition or major reconfiguration, record that difference rather than quietly comparing a smaller conversion with a larger detached ADU. Keep design, permitting, construction, utility work, and a reasonable allowance for unresolved conditions visible in both budgets.
A garage conversion may retain a slab, roof, and walls, but those components still have to work as part of a habitable unit. A detached ADU needs its own foundation and structure. In return, it can leave the garage available for the household. The financial comparison is therefore about work required and a function you may keep or lose.
Where the scope changes, phase by phase
Planning and requirements review
Both routes need a design that fits the property and a review of the rules that apply to the proposed work. For a conversion, we would establish whether the existing garage footprint and proposed openings can support the intended layout. For a detached build, we would examine placement, access, and how the new footprint interacts with the rest of the lot. A workable floor plan alone does not settle site feasibility; even a pre-approved ADU plan still has to fit the property where it will be built.
Before either budget treats a requirement as settled, check current Sunnyvale ADU and garage-conversion rules, applicable state requirements, permit review, parking provisions, and utility requirements against your address and design. Do not assume a fixed fee, that replacement parking is required, or that either proposal will receive approval without that review. As with other Bay Area remodel rules, an early requirements check can change the scope before drawings are finalized.
Site and structural assessment
The garage route gets a cost advantage only to the extent that its existing parts can be reused. We would inspect the slab for condition and suitability, look at drainage and moisture exposure, and assess framing, roof, and openings. Repairs, reinforcement, or changes to accommodate a living-space layout can consume savings that seemed obvious when looking only at the garage exterior.
The detached route begins with a new foundation, so there is no existing garage slab to adapt. Its assessment instead asks what site preparation, foundation design, drainage work, and crew or equipment access the chosen location calls for. A difficult route through a narrow side yard, for example, may affect construction methods and labor even if the proposed ADU itself is unchanged. Neither set of site costs is reliable from square footage alone.
Construction and connections
Converting a garage can mean upgrading insulation, windows, doors, and the building envelope, changing framing or openings, and building out the bathroom, kitchen, and interior. A detached unit requires the entire new shell—foundation, framing, roof, exterior finishes, and interior buildout. Holding the unit specifications steady makes it easier to see whether the existing garage actually reduces work rather than merely shifting spending toward repairs and upgrades.
Both routes need workable water, sewer, electrical, and other planned service connections. The connection points, available capacity, trenching path, and distance to the unit can differ substantially. A garage near existing services is not necessarily simple to connect; a backyard site is not necessarily difficult. We would trace the proposed routes and identify what needs utility-provider or agency confirmation before assigning those costs to either budget.
What you can choose—and what we need to check
You can set the target size, layout, finishes, preferred location, and how much garage parking or storage matters to your household. If conversion displaces those uses, decide whether you can live without them or want to budget for another storage or parking solution. That is a household choice to price, not an assumption that the city requires replacement parking.
We need to inspect or verify what preference alone cannot resolve: slab and framing condition, drainage, detached foundation needs, build access, utility routes and capacity, and the current requirements for the exact proposal. Keep these as named open items in a preliminary comparison. An uninspected slab should not appear as a confirmed saving, and an unverified utility route should not disappear into a generic allowance.
How two Sunnyvale properties could produce different answers
Illustrative property A: The garage has a sound, adaptable slab and shell, its proposed layout needs limited structural change, and the household rarely uses it for parking or storage. If inspection supports those assumptions and utility connections are practical, conversion could require less new construction than a detached unit. The budget would still include the work needed to make the space habitable; this is an example, not a Sunnyvale quote or approval prediction.
Illustrative property B: The garage needs substantial slab or framing work, and the household depends on it for a car and seasonal storage. A detached unit has a new-foundation cost, but preserving the garage may avoid a replacement-storage project and protect daily convenience. Depending on site access and utility runs, the apparent conversion savings could narrow or reverse. This result, too, remains provisional until the property and proposed scope are reviewed.
Turn the comparison into two usable budgets
Our coordinated design-and-construction scope review starts with the same unit specifications for both routes. We document what a site assessment finds, check current Sunnyvale and applicable state requirements for the proposed scope, and separate confirmed work from items that still need investigation or outside confirmation. That lets us prepare a conversion budget and a detached-build budget with their own assumptions, unresolved questions, and contingency—not one generic ADU price applied twice.
If you are weighing both options, ask IBuilders for that route-specific review before committing to detailed plans. It gives you a clearer basis for choosing the space you want and the garage function you are willing—or unwilling—to give up.
Questions worth settling before you choose
Can I compare budgets before opening up the garage?
Yes, as preliminary budgets, provided the condition of concealed or untested components is identified as an assumption. We would mark where further assessment could change the conversion scope and carry an appropriate contingency rather than count uncertain reuse as a guaranteed saving.
Does keeping the garage always justify a detached ADU?
No. Put a realistic value on the parking and storage you actually use, including any alternative you would want if you convert. Then compare that household tradeoff alongside the verified construction scopes; the answer may differ even between otherwise similar lots.