An ADU is a second self-contained home on a lot that already has one. Here is what they cost, what the law allows in your state, and what the site work adds.
The accessory dwelling unit is the fastest-growing residential category in the country, and the reason is regulatory rather than architectural. Fifteen states have passed statutes in the last several years that force local jurisdictions to permit a second unit on a single-family lot — often ministerially, often without owner-occupancy conditions, and often with parking mandates prohibited outright. That changed a niche product into a mainstream one.
Factory construction fits this category better than any other residential type. The unit is small enough to ship complete, the finish quality benefits from a controlled environment, and the on-site portion collapses from months of framing into hours of setting. What that means for a homeowner is that the permit clock and the build clock run at the same time instead of one after the other.
Accessory dwelling units are the residential type where factory construction has the clearest structural advantage, and the reasons are specific rather than promotional.
The unit is small enough to ship complete. A 385 to 1,200 square foot building fits within legal transport dimensions as one or two modules, which means the finished interior — cabinetry installed, tile set, fixtures mounted, appliances in place — arrives on a truck. Larger houses have to be broken into more modules with more field mating, which erodes the advantage.
The site work is the only weather-exposed portion. Foundation, utility trenching and the set itself are measured in days. Everything else happened indoors under controlled humidity, on a jig, with the same crew building the same assembly repeatedly. That is why blower-door numbers on factory units routinely beat site-built equivalents, and it is why stretch energy codes are easier to satisfy.
Permitting and fabrication overlap. On a site-built project nothing gets framed until the permit issues. On a factory project the module is in production while the plans examiner works, so the two clocks consume the same calendar rather than consecutive calendars. This is the single largest source of schedule compression and it is often misattributed to the factory simply working faster.
The backyard is a constrained job site. A detached unit behind an occupied house means months of trades walking past the homeowner's windows, a dumpster in the driveway and daily disruption. Setting a finished module collapses that into a single day.
Fifteen states have passed accessory dwelling legislation that overrides restrictive local zoning, and the provisions cluster around four levers. Understanding which levers your state pulled tells you most of what you need to know about the process.
Ministerial or by-right approval removes discretionary review. Instead of a conditional-use hearing where neighbors can object, the application is checked against objective standards and approved if it complies. California's 60-day ministerial clock is the strongest version of this.
Owner-occupancy prohibitions stop jurisdictions from requiring that you live on the property. This matters enormously for investors and for anyone who may relocate, and it also makes the unit financeable as an income property rather than an owner amenity.
Parking mandate limits remove the requirement to add off-street spaces, which on a typical urban lot was frequently the provision that made a unit physically impossible.
Impact fee caps reduce or eliminate per-unit development charges on small units. California prohibits them entirely under 750 square feet; Washington caps them at half the single-family rate.
| Footprint | Layout | Our turnkey | National builder published | Difference |
|---|---|---|---|---|
| 385 sq ft | Studio | $232,000 | $298,800 | $66,800 |
| 500 sq ft | 1 bed / 1 bath | $253,000 | $324,800 | $71,800 |
| 640 sq ft | 1 bed + office | $278,000 | — | — |
| 800 sq ft | 2 bed / 2 bath | $312,000 | $424,800 | $112,800 |
| 1,000 sq ft | 2 bed + great room | $341,000 | — | — |
| 1,195 sq ft | 3 bed / 3 bath | $374,000 | $498,800 | $124,800 |
Comparison column reflects a national turnkey ADU builder's own published fixed-price list as of 2026. Both columns are all-in scopes covering design, permitting, foundation, delivery, set and utility connection. Our figures are budgeting estimates; a confirmed number requires a site address.
| Finish tier | Base rate | What it includes |
|---|---|---|
| Essential | $265/sq ft | Painted drywall, LVP flooring, stock cabinetry, standard appliance package, single-hung windows |
| Signature | $325/sq ft | Quartz counters, tile wet areas, soft-close cabinetry, stainless appliances, black-framed sliding glass |
| Premium | $395/sq ft | Full-height tile, custom millwork, panel-ready appliances, wide-format glazing, cedar soffit detailing |
Fifteen states where statute, not the local planner, decides.
Slot-based, not a fixed promise.
A factory runs a queue. When you reserve a slot you are buying a position in that queue, and the honest answer to "how fast" depends on three things: how many slots are already committed, the lead time on the specific materials your configuration calls for, and whether your permit clears before your slot opens.
Current windows run 30 to 90 days from released engineering to delivery for standard configurations. Smaller units with Essential finishes sit at the fast end. Anything with a ballistic package, a hurricane-rated envelope, custom glazing or a second story sits at the slow end because those components have their own supply lead times.
The reason this beats a site-built schedule is not that the factory works faster on any given day. It is that permitting and fabrication run in parallel instead of in sequence, and the on-site set is measured in hours rather than months. Weather stops a framing crew; it does not stop a production line.
Windows move with demand and material availability. A slot quoted today holds while capacity lasts.
Tell us the scope and the ZIP. You get a real delivered-and-installed number, not a "contact us for pricing" runaround.
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