Four ways to add a unit. They are not interchangeable.
Detached, garage conversion, junior ADU, prefab. Most people arrive having decided on one because of a number they read somewhere, and change their mind once they see what the lot actually favours. Here is the honest comparison, including the part each option is worst at.
Cost below is relative between the four, not a quote — two lots on the same street differ once utilities, grade, and access come into it. We give a real number after the free site walk.
Detached ADU
A standalone home in the back yard.
Garage conversion
Turn space you already have into a permitted unit.
Junior ADU (JADU)
A small unit carved from inside the house.
Prefab ADU
Predesigned & built. Bayfront will handle site prep and installation.
Same lot in all four drawings — only the highlighted unit moves.
At a glance
| Attribute | Detached | Garage | JADU | Prefab |
|---|---|---|---|---|
| Relative cost | ||||
| Relative build time | ||||
| Typical size | Up to 850–1,200 sq ft, depending on which rule your city is under | The garage's existing footprint — not subject to unit size limits | Up to 500 sq ft | Whatever the manufacturer's models offer, within your city's cap |
| Built from | New construction, ground up | An existing garage | Existing space inside the home | A factory-built module set on a new foundation |
| Permit path | Ministerial, 60-day decision clock | Ministerial, and usually the simplest set of drawings | Ministerial, simplest path and lowest fees | Ministerial, plus the manufacturer's state-approved plans |
| What usually decides it | Whether the back yard has room once four-foot setbacks are drawn | The condition of the existing slab, framing, and roof | Whether the floor plan has space that can be separated cleanly | Whether a truck and a crane can physically reach the drop point |
A detached unit is our longest build at about 6 weeks of construction — permitting and inspection scheduling sit outside that. Every unit carries a 12-month warranty from certificate of occupancy.
Detached ADU
A separate building on its own foundation, from a compact studio up to a two- or three-bedroom cottage. It is the most private option for whoever lives in it and the most flexible to design, because nothing about the existing house constrains the floor plan. It is also the only one of the four that adds square footage without taking any away.
Best when
- Rental income is the goal and you want a full, private unit
- Family needs real separation, not a shared wall
- The lot has usable back-yard space and equipment access
Worth knowing
Our longest build, and the one where access matters most — a narrow side yard can decide the whole approach.
Garage conversion
The structure, the foundation, and often the utilities are already there, so a conversion skips the slowest and most expensive parts of a ground-up build. State law leans in its favour too: you cannot be required to replace the parking you lose, no setback applies to a unit created inside an existing structure, and unit-size, open-space and landscaping limits do not apply to converted space.
Best when
- The garage is storage rather than parking
- You want rental income on the shortest realistic timeline
- The back yard is too tight for a detached unit
Worth knowing
You give up the garage, and an older slab or roof sometimes needs work the drawings did not anticipate. We check both on the site walk.
Junior ADU (JADU)
Up to 500 square feet, created within the walls of the existing home — often a bedroom with its own entrance added, sometimes an attached garage. It is the cheapest and simplest permit of the four, and the only one that takes space from the house rather than adding it. Since January 2026, whether an owner must live on the property depends on whether the JADU has its own sanitation facilities.
Best when
- A family member needs to be close but independent
- You want the lowest-cost, fastest route to a permitted unit
- The house is larger than the household needs
Worth knowing
It comes out of your house. And if it shares sanitation facilities with the main home, an owner has to live on the property.
Prefab ADU
The unit is built in a factory and craned onto a foundation you have already permitted and poured. The on-site phase is shorter and far less weather-dependent, and the finish quality is consistent because it was not built outdoors. What it does not skip is the site work — foundation, utility connections, access, and the permit all still happen on your lot.
Best when
- Timeline matters more than a bespoke floor plan
- You want predictable finishes and fewer days of site disruption
- The lot is open enough for delivery and craning
Worth knowing
Delivery access is the constraint people underestimate. A tight or sloped lot can rule it out even when the yard looks big enough.
The lot usually picks for you.
Setbacks, access, grade, and where the sewer lateral runs decide more than preference does. We walk the property, tell you which of the four it actually favours, and why — free, and with no obligation.