Panelized ADUs: A Faster, Lower-Cost Way to Build Accessory Dwelling Units

An accessory dwelling unit is the smallest useful piece of housing supply in America. It is also one of the most expensive per square foot. The average ADU costs $180,000 to build, and most land between $40,000 and $360,000, according to Angi's 2026 ADU cost data. At $150 to $300 per square foot, a 600 square foot backyard unit can cost as much as a small house.

Panelized construction changes that math. Module Building Systems manufactures finished wall and floor panels in a 40,000 square foot CNC facility, ships them flat-packed on standard flatbeds, and assembles them on site. Project cost drops up to 30%. Delivery runs 50% faster. Waste falls by up to 90%.

What is a panelized ADU?

A panelized ADU is an accessory dwelling unit built from factory-made wall, floor, and roof panels instead of lumber cut and framed in a backyard. The panels arrive finished. Windows, doors, cladding, insulation, sheathing, and MEP are already installed before the truck leaves the plant.

This is not volumetric modular. A modular ADU ships as a finished box on an oversize load, which means route surveys, permits, and escorts. Panels ship flat and vertical on a standard flatbed. One truck carries more building and goes where a normal truck goes.

It is not a kit home either. A kit ships you sticks and a manual. MBS ships completed assemblies with the trades already done. The difference between the two panelized building products shows up in the labor bill, not the brochure.

Why ADUs cost so much per square foot

An ADU carries the fixed costs of a full house on a fraction of the floor area. Foundation, utility connections, a kitchen, a bathroom, a mechanical system, and a permit set all show up whether the unit is 400 square feet or 1,400. Labor is roughly 40% of a site-built ADU budget. Materials are another 45% to 50%.

Site work is where the money leaks. A backyard is a bad factory. Crews work around weather, tight access, neighbors, and a schedule that stretches every time a trade cannot get in.

How panelized construction cuts ADU cost up to 30%

The labor moves indoors

Framing, sheathing, insulation, window installation, cladding, and rough MEP happen at fixed stations on a factory floor. The work is repeatable and supervised. Nothing waits on rain. A smaller crew handles what is left on site, which matters when skilled trades are hard to book. See how the MBS process runs from design to assembly.

Pricing is fixed at the factory

MBS quotes panel packages at a fixed factory price. That removes the largest source of ADU budget overruns, which is not the material cost. It is the change orders and the weather days priced by the week.

Up to 90% less waste

CNC cutting optimizes every sheet and every stud. Offcuts get reused inside the plant instead of hauled to a landfill. MBS panelized construction produces up to 90% less waste than site framing. On an ADU that also means no dumpster parked in a residential driveway for four months.

The panelized ADU timeline

MBS manufactures a 1,500 square foot home in one week. On site, the panels assemble to weather-tight lockdown in 4 days. An ADU is smaller than that, so both the factory run and the lockdown compress.

Speed matters more on an ADU than on a subdivision. The unit sits in someone's yard, next to a house that people live in. A short, predictable build is the difference between a neighbor who tolerates the project and one who files a complaint.

What arrives on the truck

• Exterior wall panels with windows and doors already installed

• Cladding, insulation, and sheathing applied at the factory

• Floor and roof panels cut to the same drawing set

• Mechanical, electrical, and plumbing runs pre-installed in the panel

Everything ships flat-packed and vertically on standard flatbeds, within 1,000 miles of the MBS plant in St. Louis.

Tiny homes, backyard cottages, and carriage houses

The same panel system builds tiny homes and carriage houses. A tiny home is usually under 400 square feet and often sits on a chassis. An ADU is permanent, permitted, and tied into utilities. A carriage house is an ADU over a garage. All three gain from the same thing: the design repeats, so the factory gets faster and cheaper every time it runs that drawing.

That repeatability is why panels beat one-off site framing on small units. A 700 square foot backyard cottage does not have enough square footage to absorb a custom build.

States are legalizing ADUs faster than the industry can build them

Eighteen states now have laws that broadly legalize accessory dwelling units, according to the Mercatus Center's taxonomy of state ADU laws. Eleven of them passed in the past four years. Zoning is no longer the binding constraint in those states. Cost and capacity are.

Legalizing an ADU does not build one. Someone still has to deliver a permitted, finished unit at a price a homeowner, a land bank, or a nonprofit developer can carry. That is the gap panelized construction closes.

Who MBS builds ADUs for

Developers adding units on infill lots. Municipalities and land banks unlocking backyard density. Community development corporations building attainable housing. General contractors who want a fixed-price shell and a four-day lockdown instead of a framing crew they cannot staff. The full range of MBS panelized solutions runs from ADUs and single-family homes through townhomes, mixed-use, and 1 to 6 story multi-residential.

MBS was founded in 2023 in St. Louis around affordable housing, homelessness, and disaster relief. ADUs fit that mission cleanly. They add a home to a block that already has streets, sewer, and schools, without asking a city to build anything new. Read more about MBS.

Panelized ADU questions, answered

What is a panelized ADU?

A panelized ADU is an accessory dwelling unit built from factory-made wall, floor, and roof panels instead of lumber framed on site. MBS panels arrive finished, with windows, doors, cladding, insulation, sheathing, and MEP already installed, then assemble on a prepared foundation.

How much does a panelized ADU cost?

Site-built ADUs average $180,000 nationally and range from $40,000 to $360,000. MBS panelized construction cuts total project cost by up to 30% and quotes a fixed factory price for the panel package, which removes the weather delays and change orders that drive ADU overruns.

How long does it take to build a panelized ADU?

MBS manufactures a 1,500 square foot home in one week and assembles it on site to weather-tight lockdown in 4 days. Most ADUs are smaller, so both stages compress. Overall delivery runs about 50% faster than site-built.

Is a panelized ADU code compliant?

Yes. Panels are engineered and manufactured to the building code in force where the ADU is built, and the finished unit is permitted and inspected by the local authority having jurisdiction, the same as a site-built home. More answers are on the MBS FAQ.

How far can MBS ship ADU panels?

MBS ships panels flat-packed and vertically on standard flatbeds within 1,000 miles of its 40,000 square foot CNC manufacturing facility at 921 N 20th Street, St. Louis, Missouri. No oversize load permits or route escorts are required.

Build your ADU with MBS

Send us the lot, the unit size, and the timeline. We will price the panel package at a fixed factory rate. Get a quote or email info@modulebs.com.

Module Building Systems. A Better Way to Build for Tomorrow. Advanced Panelized Construction. 921 N 20th Street, St. Louis, MO 63106.