Modular ADU: Costs, Builders, and What to Know

A modular ADU is a second home built in a factory, trucked to your property in finished sections, and craned onto a foundation in about a day. It meets the same building codes as the main house, so the city treats it like any addition and a bank treats it like real property. The factory ships it 70 to 90% complete, plumbing and wiring already in the walls, which is why the part of the job that happens on your lot takes days instead of months.

That is the whole appeal, and also where the confusion starts. Modular gets lumped in with manufactured homes, prefab kits, and tiny homes, and the differences decide what you can build, what it costs, and whether anyone will lend against it. So start with the words.

What a modular ADU is, and what it is not

An accessory dwelling unit is any second home on a lot that already has a house. A converted garage counts. A detached cottage in the backyard counts. Modular describes one way to build that unit: in a factory, as boxes, finished inside before they leave the floor.

The line that matters runs between modular and manufactured. A modular ADU is built to the International Residential Code, the same prescriptive code your house was built to, and it sits on a permanent foundation poured on your lot. A manufactured home is built to the federal HUD Code on a permanent steel chassis, a different standard with different rules. Factory construction is the only thing they share.

Everything downstream flows from that one distinction. Because modular meets local code, the city permits it like a site built addition, the appraiser values it like one, and a mortgage lender will treat it as part of the real property. Manufactured homes hit zoning limits in some residential areas and struggle to qualify for standard mortgages, especially on leased land. Same factory, very different paperwork.

A third method sits between modular and traditional building: panelized kits. The factory makes flat structural panels, ships them flat, and a local crew assembles them on site. More site labor than modular, more flexibility, lower delivery cost because nothing arrives as a finished box that needs a crane.

Modular, kit, or stick built: which suits your lot

Three ways to put an ADU in your yard, and the right one depends more on your lot than your budget.

Modular wins on speed and a predictable price. The factory builds to a set plan, so you trade deep customization for a number you can rely on and a timeline measured in months. Build cost lands around $300 per square foot before site work.

Stick built wins on flexibility. A crew framing on site can follow a sloped lot, an odd shape, or a tight setback that no factory box would fit. You pay for it, in both money and time: $300 to $450 plus per square foot and 28 to 40 weeks of on site work, against 12 to 14 weeks for a modular install.

Panelized kits are the middle path. Lower delivery cost, some room to customize, and a route that suits owners willing to manage a local contractor rather than buy a finished box. Cost runs near manufactured at the unit level, but the total depends heavily on who you hire to stand it up.

Factor	Modular	Panelized kit	Stick built
Built where	Factory	Factory panels, assembled on site	Entirely on site
Build cost per sq ft	~$300	~$275 to $325	$300 to $450+
On site time	12 to 14 weeks	16 to 24 weeks	28 to 40+ weeks
Design flexibility	Low to medium	Medium	High
Building code	IRC (local)	IRC (local)	IRC (local)
Financing	Standard mortgage	Standard mortgage	Standard mortgage
Best for	Speed, fixed budget	Cost control, some DIY	Complex lots, full custom

One thing the table hides: modular needs a crane, and a crane needs access. A narrow side yard, low power lines, or a backyard you can only reach through the house can turn a clean install into a logistics problem, or rule modular out entirely. Check the access before you fall for the floor plan.

How much a modular ADU costs

A modular ADU typically runs $150,000 to $300,000 all in for a 400 to 800 square foot unit nationally, and $200,000 to $350,000 in California once you add site prep, utilities, foundation, delivery, and permits. The unit price a builder quotes, often $50,000 to $150,000, is the box, not the project. Budget another $50,000 to $100,000 for everything that turns the box into a home you can live in.

That gap between unit price and all in cost is where most budgets break. A marketplace listing might advertise a unit at $50,000 to $250,000, but that is the purchase price of the module, not the installed cost. On a California project, the numbers that actually leave your account look more like this:

• Prefab unit: $120,000 to $200,000
• Site preparation: $10,000 to $40,000
• Utility hookups: $15,000 to $30,000
• Permits and city fees: $5,000 to $15,000
• Installation and finishing: $20,000 to $40,000

By size, national figures land around $60,000 to $120,000 for 400 square feet, $90,000 to $180,000 for 600, and $150,000 to $300,000 for a full 1,000 square foot two bedroom. California runs higher at every size. A studio there starts near $150,000 all in and a two bedroom clears $300,000 without much effort.

Per square foot tells the same story faster. Figure $150 to $300 across most of the country, and $300 to $600 in California coastal markets. Texas sits at the low end, $150 to $300. The San Francisco Bay Area sits at the top, where all in projects run $210,000 to $630,000.

What pushes a project toward the top of its range: a hillside or sloped lot that needs foundation engineering and tricky crane access, old utility infrastructure that has to be upgraded before you connect, a permitting jurisdiction that moves slowly, and finishes or solar that you chose yourself. None of these are exotic. Most projects hit at least one.

From permit to move in: the timeline

Most modular ADU projects run 4 to 6 months from deposit to move in. Stick built the same unit and you are looking at 12 to 18 months. The reason is not that factories work faster, though they do. It is that the factory builds your modules while your site is being prepared, so two phases that happen one after another in stick build happen at the same time in modular.

The sequence runs roughly like this. Design and a site survey come first, then permit submission. Once the permit clears, the factory starts the modules, six to eight weeks of build, while a local crew pours your foundation and trenches utilities. Delivery and the crane lift take about a day. Then connections and final inspections, which add four to eight weeks depending on your utility provider.

Permitting is the variable that swings the whole timeline. In California, existing law caps standard ADU review at 60 days, and AB 1332 added a 30-day pathway for applications using preapproved plans. As of 2026, SB 543 forces the agency to confirm a complete application within 15 business days or the application is deemed complete automatically. That is the fastest a modular ADU has ever moved through a California permit office. Outside California, add one to three months and check with the building department before you commit to a schedule.

Do you need a permit, and what varies by state

Yes, always. A modular ADU needs a building permit everywhere. The good news is that modular meets local IRC code, so the review runs like any site built addition rather than the harder zoning fight manufactured homes face.

Where you build decides how easy that review is. California leads the country by a wide margin. AB 976 removed owner occupancy requirements for standard ADUs in 2024, though junior ADUs remain subject to it. State law caps rear and side setbacks at 4 feet, allows units up to 800 square feet and 16 feet tall by right, exempts ADUs of 750 square feet or less from impact fees, and drops the extra parking requirement within half a mile of transit. SB 1211 even allows up to eight ADUs on a multifamily parcel.

The rest of the country is a patchwork. Oregon requires cities to allow one ADU per single-family lot and caps detached units at 800 square feet. Washington’s HB 1337 is among the strongest laws anywhere, with no owner occupancy requirement and the right to sell an ADU separately from the main house. In 2026 Florida advanced a statewide ADU mandate through its legislature, among the first Southern states to do so. Texas has no state law at all, though Austin’s HOME initiative and Houston’s loose zoning make both cities friendly in practice. Vermont, Maine, Connecticut, Minnesota, and Montana round out the permissive end.

Two things still trip people up even in friendly states. An HOA can ban ADUs on your street even where the city allows them. And a few jurisdictions hold onto minimum lot sizes or stricter setbacks than the state floor. Confirm both with your local building department before you sign anything.

Modular ADU builders worth knowing

Most of these companies quote per project rather than post a price, and the ones that do publish a number are the ones worth anchoring to. Anchor to those.

California has the deepest bench. Abodu sells turnkey units from $278,800 with fixed pricing across six plans, and notes permits and fees average around $17,000 on top, varying by city. LiveLarge Home starts at $412 per square foot for steel framed, Title 24 compliant units, with a four to six week on site install. Samara, the spinoff that came out of Airbnb, prices its units from $152,000 before installation, with all in costs estimated at $622 to $800 or more per square foot in third party analysis. Villa Homes does panelized turnkey in the Bay Area, around $300,000 for an 800 square foot two bedroom. ADORE Homes builds factory turnkey units across the Bay Area on a quote basis.

Outside California the field thins but the options are real. Studio Shed, based in Colorado and serving multiple states, leans DIY friendly with modular kits from 250 to 1,000 square feet, roughly $240,000 all in for a 476 square foot unit. Mighty Small Homes ships panelized kits nationally from its Louisville base, 192 to 2,108 square feet, with the buyer handling foundation, utilities, and finishes locally. DC Structures, out of Oregon, ships prefab ADU kits to every state. In the Northeast, Westchester Modular builds custom design build units in New York with a basement option, and Backyard ADUs covers Maine and Massachusetts end to end. Makara Builders leads ADU work across Maryland, DC, and Virginia, and Little Home Builder handles Colorado.

The price per square foot, where builders publish it, lands like this:

Builder	Region	Type	Price signal
Abodu	California	Turnkey	From $278,800 fixed, ~$17K permits extra
LiveLarge Home	California	Turnkey, steel frame	From $412/sq ft
Villa Homes	SF Bay Area	Panelized turnkey	~$375/sq ft (800 sq ft 2BR)
Samara	California	Turnkey	$622 to $800+/sq ft all in
Studio Shed	Colorado, multi state	Modular kit	~$504/sq ft all in (476 sq ft)

The small units skew high per square foot, which is normal: a kitchen and a bathroom cost about the same whether they sit in 400 square feet or 800, so the fixed cost spreads thinner as the unit grows. Read the all in number, not the unit price, and read it against your own size.

You can browse modular home models and floor plans on Prefab Market to see what a finished unit looks like at different sizes, and check the manufacturers listed on the directory for current makers.

How financing works

Lenders treat a modular ADU as real property, the same as a site built home, because it meets local IRC code. That single fact opens up financing that manufactured homes often cannot reach. Three routes cover most buyers.

A cash out refinance replaces your current mortgage with a larger one and hands you the difference in cash, usually up to 80% of your home’s current value. It works if you have the equity already. A construction loan suits a bigger build than your equity can cover, releasing funds as the work hits verified milestones, and modular usually qualifies on the same terms as stick built. ADU specific lenders like RenoFi take a different angle: they lend against what your home will be worth after the ADU is finished, not what it is worth today, which stretches how much you can borrow before a single module is built.

Plan on a 680 FICO for a cash out refi and 700 for a construction loan or a RenoFi product. And budget for one quirk of buying a box: the builder wants a large deposit before the unit ships, because they build it before it ever reaches your lot. Stick built lets you pay in stages as the work happens. Modular asks for money up front. Even with a loan approved, that deposit comes out of your pocket first.

An ADU pays its way over time. The average property value bump from an ADU runs about 35%, and in quality markets, a detached unit can rent for roughly $39,000 a year. Whether that math works for you depends on your market and your rate. But of the ways to add a rentable, mortgageable second home to your lot in under six months, modular is the one with the fewest surprises.

Ready to compare? Browse modular home models on Prefab Market to start matching size and layout to your budget.