Prefab Granny Flats (ADUs): 2026 US Buyer's Guide with Real Prices

A prefab granny flat is a factory built secondary home that sits on the same lot as a main house. In the United States these are called ADUs, accessory dwelling units. The term granny flat comes from Britain and Australia; American planning codes use ADU. Prefab means the major construction happens off site, in a factory, and the finished unit or its parts are delivered and installed on your property.

The reason buyers get burned is the gap between two numbers. A manufacturer advertises a $37,000 kit. The same project costs $150,000 by the time it has a foundation, utility hookups, and a certificate of occupancy. Neither figure is wrong. They describe different products. This guide separates the unit price from the all in price across the real builders, lays out the four ways these homes get made, and covers the state rules that decide whether you can build one at all.

Prices below were verified in June 2026 from each manufacturer’s own pages.

At a glance

• Cheapest unit: PMHI panelized kits from $37,053 for 616 sq ft (57 sq m), materials only.
• Cheapest complete install: Abodu Studio at $278,800 all in, California, including foundation, permits, and delivery.
• Four build systems: panelized kit, modular, manufactured (HUD code), and shipping container. They differ on financing and zoning, not just price.
• All in budget, typical 500 to 800 sq ft unit: $150,000 to $400,000+, with California at the high end.
• Timeline: 6 to 12 months for most projects. The unit installs in days; permitting is the slow part.
• Where it is easiest: California, Oregon, Washington, and Massachusetts have state laws that force cities to allow ADUs.

What a prefab granny flat is, and why Americans say ADU

An ADU is a self contained second home on a lot that already has a main house. It has its own kitchen, bathroom, and entrance. The four common forms are detached (a separate structure in the backyard), attached (sharing a wall with the main house), a garage or basement conversion, and a junior ADU, which in California is a converted room inside the main house capped at 500 square feet (46 square meters).

There is no federal definition of an ADU. Every rule that matters is set by your state and your city. That is why two identical backyards, one in Sacramento and one in Atlanta, can have completely different answers to the same question.

Prefab is a loose word in this market. It covers anything where significant building happens in a factory before the unit reaches your lot. That spans four genuinely different systems, and the difference is not cosmetic. It changes how you finance the home and whether your city will permit it at all.

The four ways a prefab granny flat gets built

Ask any manufacturer one question before you sign: which of these four systems do you build? Marketing copy blurs them on purpose. The answer decides your loan options and your zoning fight.

Panelized kits arrive as flat wall, floor, and roof sections that a local crew assembles on your foundation. They are built to the International Residential Code, the same code as a stick built house, so the finished home is real property and finances normally. PMHI is the classic example: the kit ships nationwide and you arrange your own contractor for everything else.

Modular units are built as three dimensional boxes in a factory, trucked to your lot, and craned onto a permanent foundation. They are also IRC built and treated as real property, identical to site built for zoning and lending. Abodu builds this way and sets the unit in about a day. You pay more than a kit, but a far larger share of the work is done.

Manufactured homes are built entirely in a factory on a steel chassis to a federal HUD standard (24 CFR Part 3280), not the IRC. That single fact creates two problems. Financing is harder, because most lenders treat the home as personal property and conventional mortgages are largely off the table; you are looking at chattel loans or FHA Title I at higher rates. And zoning is restrictive, because many cities bar manufactured homes from residential lots or exclude them from ADU eligibility. The per square foot price is the lowest of the four, but appraisal and financing penalties can eat the saving.

Shipping container conversions have no uniform federal code, so compliance rests entirely on your local jurisdiction. California and Oregon permit them as ADUs when they meet local building standards. Plenty of cities ban them or demand expensive structural changes. Standard ADU lenders mostly will not touch them. A basic converted unit can start around $35,000 to $80,000, but code compliance and site work add up fast.

Build system	Code standard	Property type	Zoning treatment	Financing	All in cost	Typical timeline
Panelized kit	IRC / IBC	Real property	Same as site built	Standard	$150k to $300k	6 to 9 months
Modular	IRC / IBC	Real property	Same as site built	Standard	$200k to $450k	6 to 12 months
Manufactured (HUD)	HUD federal code	Usually personal	Often restricted	Chattel / FHA Title I	$120k to $280k	4 to 8 months
Shipping container	Local codes only	Real if on a foundation	Highly variable	Non standard	$100k to $250k+	6 to 12 months

The honest takeaway from builders who run the numbers is that the price gap between systems narrows once permitting, site work, utilities, and finishing are added to every option. A cheap unit on a hard site can finish higher than a pricier unit on an easy one. For more on where modular and manufactured part ways, see our guide on modular vs manufactured homes, and for containers specifically, modular vs container.

What a prefab granny flat costs in 2026

Two prices, always. The unit price is what the factory charges for the structure. The all in price is what you pay to live in it: unit, foundation, delivery, utility connections, permits, and finishing. Most buyer confusion comes from comparing one manufacturer’s unit price against another’s all in price.

Here is what the real builders charge, verified in June 2026.

PMHI (Pacific Modern Homes) sells panelized granny flat kits, materials only. Meadow View runs $37,053 for 616 square feet (57 square meters), one bed, one bath. Maywood is $37,417 for 600 square feet (56 square meters). The larger Camino is $60,017 for 994 square feet (92 square meters). PMHI states a kit range of $25,000 to $75,000 across the line. None of that includes the foundation, labor, permits, site prep, or utility hookups, all of which you arrange yourself.

Studio Home sells a more complete panelized system, unit price only. The Summit series runs from $119,917 for 476 square feet (44 square meters) up to $155,426 for 800 square feet (74 square meters), and the Laurel ADU line starts at $119,500. Those reflected a 15% summer promotion; list prices run roughly 15% higher. Site prep, foundation, permits, and utility connections are still on you.

Abodu sells modular units all in, and its pricing is the strongest complete data available, with the caveat that it operates in California only as of June 2026. California labor costs are high, so treat these as a ceiling rather than a national figure. The Abodu Studio starts at $278,800 and averages $300,500 finished for 340 square feet (32 square meters). The Abodu One is $326,800 base for 500 square feet (46 square meters). The two bedroom Abodu Two+ reaches $478,800 average for 800 square feet (74 square meters). That includes foundation, permits, delivery, installation, and standard utility connections, but excludes permit fees and taxes, which average around $17,000, plus extras like long utility trenching or tree removal.

The contrast between a $37,000 PMHI kit and a $278,800 Abodu install is not one company overcharging. PMHI sells you materials and you build the project. Abodu sells you a finished home with nothing left to coordinate. Decide which job you want before you compare prices.

For a complete project, the components stack up roughly like this:

Component	Low	High	What drives it
Prefab unit (kit or modular)	$37,000	$200,000	Kit excludes labor; modular includes far more
Site prep and foundation	$10,000	$40,000	Slope, soil, and drainage
Delivery and installation	$5,000	$30,000	Crane access difficulty
Utility connections	$15,000	$30,000	Distance from the mains
Permits and impact fees	$5,000	$25,000	California waives impact fees under 750 sq ft
Finishing (where not included)	$10,000	$40,000	Kits need more; modular needs less
All in, typical 500 to 800 sq ft unit	$150,000	$400,000+	California skews high; TX, OR, WA lower

On a per square foot basis the unit alone runs $80 to $300 depending on system, while the installed all in figure lands at $200 to $450. A site built ADU costs $250 to $450+ per square foot, so prefab does save money, just less than the unit only sticker suggests. The site prep line is where budgets break. Our guide to the hidden costs of prefab goes through the surprises in detail.

State rules that decide whether you can build at all

Before price, before model, the question is whether your jurisdiction will let you. Four states have made it straightforward by overriding local zoning. Most have not.

California is the most permissive in the country. ADUs are allowed up to 1,200 square feet with 4 foot setbacks, owner occupancy requirements have been removed statewide, and impact fees are waived for units under 750 square feet (70 square meters). The 2026 HCD ADU handbook forces local governments to approve a complete application within 60 days. SB 9, combined with an urban lot split, can allow up to four units across two parcels on what was a single family lot. If you can build an ADU anywhere easily, it is here.

Oregon requires cities over 2,500 people to allow detached ADUs by right in single family zones, with no owner occupancy rule inside urban growth boundaries. Size limits are left to each city.

Washington passed HB 1337 in 2023, requiring cities and counties that plan under the Growth Management Act to allow at least two ADUs per lot and removing owner occupancy requirements within urban growth areas. Cities must allow ADUs of up to at least 1,000 square feet. The Pacific Northwest term you will hear is DADU, detached accessory dwelling unit.

Massachusetts passed a 2025 law allowing ADUs by right in any district that permits single family homes, overriding restrictive local rules.

Texas has no state preemption, but unincorporated areas are generally ADU friendly by right and the big cities, Austin, Houston, and Dallas, have adopted permissive ordinances. Several more states, including Minnesota, Vermont, Maine, Connecticut, and Montana, have passed ADU friendly legislation since 2024.

Everywhere else, the city decides, and the rules vary wildly. The variables that will make or break your project are below. Get answers from your planning department before you fall in love with a floor plan.

What to check	The question to ask locally
Minimum lot size	How large does my lot have to be?
Setbacks	How far from the property line must the unit sit?
Owner occupancy	Do I have to live on the property?
Maximum size	What is the largest unit I can build?
Utility hookups	Can I share water and sewer with the main house?
Impact fees	What fees apply, and is anything waived under 750 sq ft?
Short term rental	Can I list the unit on Airbnb?

That last one catches people. Many cities that happily permit an ADU for family or a long term tenant ban short term rental in it outright.

The builders worth comparing

No site in the top search results gives a side by side read on these companies with a real verdict attached. Here is ours. The three below are genuine manufacturers with verified pricing. Two other names that show up in searches, Wolfind and Maxablespace, are a lead generation service and a California consulting firm respectively, not manufacturers, so they are not in this table.

Builder	System	Size range	Price	What you get	Where
PMHI	Panelized kit	600 to 1,196 sq ft	$37k to $75k, kit only	Materials; you manage all site work	Ships nationwide
Studio Home	Panelized	476 to 800 sq ft	$119,500 to $155,426, unit only	A more complete panel system, configurator	Multiple states (confirm)
Abodu	Modular	340 to 800 sq ft	$278,800 to $498,500, all in (Studio to Dwell House)	Full service, permits to install	California

PMHI is the lowest cost entry into a full size ADU, and the most work. You get a kit and a set of plans. You hire the contractor, pour the foundation, pull the permits, and finish the unit. Best for buyers who have a trusted local builder or genuine construction management ability, and for rural sites where contractor networks are thin and a kit is the practical option. Not for anyone who wants a hand to hold.

Studio Home sits in the middle. The panel system is more complete than a bare kit, with a design configurator and four architectural styles across the Laurel line. You still arrange site prep, foundation, permits, and utilities. Best for buyers who want more design control than a fixed modular box and more factory quality than a full custom build. Read the unit price as the start, not the finish.

Abodu is the most expensive and the least hassle. A project manager runs the whole thing, the plans are preapproved, and the unit is delivered and set for you. Best for California homeowners who want a finished outcome with no contractor to chase, including fire rebuilds. The constraint is the state line: as of June 2026 it builds in California only, so for buyers elsewhere it is a benchmark, not an option.

To see how prefab makers compare more broadly, browse the manufacturer directory and the full prefab catalog. Note that the directory currently lists European models for category context; the US ADU builders above are covered in this guide rather than the listings.

From order to move in

The unit is the fast part. Everything around it is the timeline. A typical project moves through eight stages.

A site assessment comes first, where the builder checks utility distances, crane access, and soil. This is where hidden costs surface, which is the good time for them to surface. Design and configuration follows, picking the model and finishes. Then the permit application, which is the longest wait: 4 to 12 weeks, sometimes more, though California’s 60 day clock and preapproved plan sets can compress it.

Factory production runs in parallel with permitting, usually 6 to 14 weeks, and that overlap is the real time advantage of prefab. While the factory builds, site prep happens on your lot: foundation, trenching, grading. A slab cures in three to four weeks; helical piles bear load in days. Delivery and installation is quick, a few days for panels or about a day to crane a modular unit into place. Utility connections take one to three weeks, driven mostly by distance to the mains. Finally a building inspection issues the certificate of occupancy.

Scenario	Total timeline
Optimized California project, preapproved plans, fast track permit	6 to 9 months
Typical US prefab project	9 to 12 months
Restrictive jurisdiction, long permit queue	12 to 18 months

The overruns are predictable. Site prep surprises lead the list: slope, bad soil, buried utilities, poor drainage. Carry $10,000 to $25,000 of contingency on any lot you have not had assessed. Utility trenching beyond the usual 50 foot allowance adds up on a large lot. Difficult crane access on a narrow or steep site adds mobilization cost. Impact fees can reach $25,000 in some cities. And base prices assume builder grade finishes; upgrades on a managed build can add tens of thousands.

Five situations, and what to build for each

The right unit depends on what you are doing with it.

Aging parent or relative. Go modular or higher end panelized, built to the IRC and designed for accessibility: single level, wider doorways at 32 to 36 inches, a roll in shower, close to the main house. Budget $200,000 to $350,000 all in for a one bed, one bath in most markets, more in California. Ask for ADA adaptable floor plans.

Long term rental. Modular or solid panelized again, for durability and because real property financing keeps your out of pocket lower. A one or two bedroom that attracts good tenants runs $250,000 to $400,000 all in. Rents range widely by market, and break even on a $300,000 build typically lands around 5 to 7 years. Check whether your city imposes owner occupancy, which kills some rental plans.

Short term rental. Same build cost as a long term unit, but read your local ordinance first. San Francisco, Santa Monica, and many other cities restrict or ban short term rental in ADUs. Where it is allowed and well run, income potential is higher, but so is the management load and the licensing.

Home office or studio. If the structure is not a habitable dwelling, no kitchen, no bedroom, permitting can be simpler, though you must confirm that locally. A small panelized kit, $35,000 to $65,000 for the unit, is the cheapest route. Add utilities and site prep and a realistic finished figure is $60,000 to $130,000. Finishes can stay basic.

Guest suite. Like the office case but built to habitable standards, so a one bed, one bath in the $200,000 to $320,000 all in range. No rental income, but a well built ADU reliably adds resale value, often a meaningful share of its cost in strong markets.

Across all five, the build system you choose flows from the use. A rental you will keep for decades justifies modular. A backyard studio does not.

Questions buyers ask

Is a prefab granny flat cheaper than a site built one? Yes, but by less than the unit price suggests. Prefab units run cheaper per square foot, and factory build is faster, but once you add the same foundation, permits, utilities, and site work that a site built ADU needs, the saving is real but modest, often 10 to 20% per square foot rather than half.

Can I get a mortgage on a prefab ADU? It depends on the system. Panelized and modular units are real property and finance like any house. Manufactured HUD code homes are harder, usually requiring chattel or FHA Title I loans. Containers are mostly outside standard lending. If financing matters to you, that alone may decide the build system. Our guide on whether modular homes are mortgageable covers the lender side.

Are prefab granny flats legal everywhere in the US? No. There is no national rule. Four states force cities to allow ADUs; most leave it local, and some local rules are restrictive enough to make a project impractical. Always confirm with your planning department first.

What size can I build? Common ADUs run from about 340 square feet (32 square meters) to 1,200 square feet (111 square meters). California caps detached ADUs at 1,200 square feet; other states set their own limits or leave it to cities.