ADU Prefab: Plan Slow, Build Fast

For Bay Area, California
Many options exist for buying a prefab home. The best option for you will depend on your specific needs and preferences. And on your property.

Why buy a prefab home?

There are several reasons why people buy prefab homes, or prefabricated homes. 
1. Perceived affordability: Prefab homes can be more affordable than traditional site-built homes. The construction process is often more efficient. The factories buy materials in bulk. The result is lower costs for the homeowner. You have to make sure though that these savings are not eaten up by transportation and crane costs.
2. Limited Customization: Many prefab home manufacturers offer a range of customization options. It allows to choose the layout and finishes, that best suit their needs and preferences. For people who find construciton overwhelming this restricted selection helps make decisions.
3. Speed of construction: Prefab homes are built in a factory. The on site construction process can be completed more quickly than traditional site-built homes. This can be beneficial for those who need to move into their new home quickly – make sure the unit is pre-approved by the State. Otherwise, it will take the same amount of time as a custom house. Prefab option also serves well those who are sensitive to construction noise.
4. Quality control: Prefab homes are built in a controlled environment. Often to the same plans – over and over again. This translates into higher quality construction and fewer defects than traditional site-built homes. You have to make sure the quality control happens in person, not virtually.
5. Sustainability: Prefab homes can be designed to be energy-efficient and environmentally friendly. Common features are solar panels, high-efficiency insulation, LED lights and low-flow plumbing fixtures. Check Title 24 to confirm. Not all prefab homes are created equal.
The same reasons cause people to look at prefab options when building an ADU. After all, an ADU prefab significantly reduces time of construction in your backyard.
Prefabricated ADUs can come in different sizes and styles. They can be customized to fit the needs and preferences of the person who wants to live in them. They are used as a place for someone to live independently. It can be an adult child or elderly parent, or it may become a rental property to generate extra income.
Do not be afraid to spend more time selecting and planning, after all it is a big ticket item that you won’t be able to return easily. Even if you bought your home kit on Amazon!
Also remember, not all prefab ADUs are modular ADUs. However, all modular ADUs are pre-fab!
prefab home log kit from Finland

Perfect Design

Design of your ADU should reflect the situation in your backyard – from where is the best view to where is the best exit.

plan for 800 sq ft house

Carefully Planned

The layout should make sense – if it has to wrap around the main house or perfectly mirror it, so be it.

prefab Connect home

Smartly Execute

Even if you bought a modular ADU prefab, it still needs foundation, utility connections and deck to become a comfortable home.

Why prefab homes are better?

First, they can be built more quickly than traditional homes. In a factory, the construction process is more efficient. Building materials are purchased in bulk often directly from manufacturers. This can be especially helpful for those who need to move into their new home quickly or who want to save money on construction costs. Your home is built at the same time as the local crew work on foundation.
Second, prefab homes are often of high quality. The controlled factory environment means less risk of weather damage. Production of the same units means less construction mistakes. The supervised construction process allows for better quality control. This means a more durable and long-lasting home.
Third, prefab homes are more environmentally friendly. The construction process often results in less waste than traditional site-built homes. This is an important consideration for those concerned about their environmental impact.

Types of  prefab homes and ADUs 

Prefab homes and ADUs offer a range of benefits that make them an attractive option for many people. Cost savings, environmental sustainability, customization options make prefab homes a great choice.
There are several types of prefabricated homes and ADU prefabs. Each comes with its own unique design implications and construction process.
1. Modular homes: Factories build modular homes and modular ADUs in sections, or modules, and then transportthem to the site. There they are assembled together to create a finished home. Modular homes can be built in a variety of styles and sizes. They can be customized to fit the needs and preferences of the homeowner. Most factories have their own base models that will be cheaper than custom options. A modular home is built to the building code of the location where it will be placed.
2. Panelized homes: Panel manufacturers built panels or sections in a factory. Then they transport the panels to the site where the local crew assembles the panels to make a new building. The panels are typically made of wood or steel and pre-cut to fit the specific design of the home.
3. Manufactured homes: Manufactured homes are built on a steel I-beam frame, with axles and tires attached underneath. They are transported to the site on their own wheels. The wheels are typically removed when the home is installed in place. They are often less expensive than modular or panelized homes, but they may have less layout and customization options. These homes are built to a national building code established by the U.S. Department of Housing and Urban Development (HUD) in 1976. The code is regularly upgraded. The home will have a HUD Data Plate. It lists all the construction performance specs of the home. There will be an exterior HUD Tag riveted to the long side of each section of the home.
4. Kit homes: Kit homes are prefabricated homes that are shipped to the site in pieces. The local crew then assembles it without wasting time on measuring and cutting on site. They come with pre-cut to plans materials and lego-like instructions for assembly. Manufacturers of kit homes offer unlimited customization options.
5. Container homes: Container homes are made from shipping containers that are repurposed and converted into living spaces. They can be more affordable, fire resistant and eco-friendly. Those built by owner-builders are typically smaller than traditional homes. There are factories that can build both main house and ADU prefab with a container structure.
The right type of prefabricated home for you will depend on your budget, lifestyle, and design preferences.
Methods we have tried over the years

ADU Prefab Options

ADU prefab wood panels
Panelized Construction

Wood panels

Every wall becomes a big wooden frame, several smaller panels fit into this big frame to become a wall. 

adu prefab with MnmMod
Panelized Construction

Steel Frames

These function same way as wood panels, but are heavier plus fire & termite resistant.

Steelblox Container ADU prefab
Modular Construction

Container Homes

A container solutions makes it possible to bring you a 1000 sq ft furnished home in 4 containers to be stitched up on site within days. Deck excluded.

ADU prefab transport
Modular Construction

Built in modular or manufactured home factories, these are similar to custom built homes – they come in fixed floor plans though with minimal customizations available.

Prefab Home Kits

One of the most comfortable to live in home types,  environmentally friendly, it can come as a home kit with every log labeled.

prefab home kit shipment
Prefab Home kits

The closest to custom built homes, the pre-cut lumber arrives packaged and numbered to be assembled according to Lego like instructions.

Why prefab ADUs are better?

ADU stands for “accessory dwelling unit” – a separate living space on the same property as a larger main house.
A prefab ADU is built in the factory – often multiple times and to State approved plans. It is then shipped to the site where it gets assembled together like a puzzle. This makes building an ADU faster and easier than a traditional ADU “stick built” on-site.
An ADU prefab significantly reduces time of construction in your backyard. 
1000 sq ft Panelized Prefab ADU - pre-approved in San Jose

Permit-ready ADU Floor Plan

Pre-approved master plan in San Jose (1000 sf)
Trade off for a Custom Floor Plan

What fun will you miss with modular construction?



The material will arrive cut to measure. the crew just needs to follow instructions and numbers on the panels to assemble the house.

drywall dust while finishing drywall before painting it
Time gains

Drywall prep dust

You won’t have to deal with dust. The ADU prefab will arrive painted. There may be some cracks caused by transportation, but they are minimal.

waterproofing shower space
Time gains


The bathrooms will be ready to use as soon as your local plumber connects the unit to the water and sewer or septic lines.

selecting grout to match tile color
No Analysis Paralysis

Color selections

Most modular companies have pre-designed sets of color combination. You do not have to sweat the details.

measuring sink
No Analysis Paralysis

Making sure the fixtures work together

You won’t have to make sure the sink is of the right size and the faucet had enough reach for comfortable use. 

tile and countertops
Stress level down

Control of lead times

You do not have to make sure the appliances, fixtures and finishes arrive on time for different trades. It’s the factory’s headache. 

Other floor plans available

All our 300+ plans can be built as ADU prefab

There is a number of pre-designed floor plans in our library – we have I shaped ADU, L shaped ADU and more.

Every thousand dollars matters when you look at an ADU as an investment into your property. This is why we try to save you money with pre-designed plans. 

It is also a reason why we consider many parameters of your site and personal requirements when giving you a recommendation. As you know now, prefab ADUs come in different shapes, sizes and materials. There are multiple options for ADU prefab foundations and  exterior looks. Not every one of them will work for your backyard.


Click to select the right model
Reality Check

Here is what you cannot escape whatever type of prefab you select.

erosion control with straw mats
Civil Engineering

Erosion Control

No run off is allowed off site. Every City and County have their own version of “Clean Bay” to follow.

Civil Engineering

Grading for Foundation

All permanent homes need foundation. Foundation requires trenching and grading. No escape.

septic system set up
Public Works

Confirming size of septic

Your septic has to be big enough for additional usage. Your sewer line has to be wide enough for more volume. You will have to comply.

fire alarm system with sprinklers on a house
Fire Department

Fire Sprinklers

If the main house has sprinklers, your ADU will need sprinklers too. 

panel electrical showing meter and breakers
Building Department & PG&E

Confirming Size of Panel

Your new ADU will be all electric. This means the panel needs enough amperage and breakers to accommodate.

ADU with big patio door, dark window frames, wood plank accents and stucco exterior


Most of prefabs are built with raised foundation. Most homeowners want sliding doors for better living. This means deck has to be accounted for.

Small Prefab ADU

Permit-ready ADU Floor Plan

narrow ADU with one bedroom

Where to buy a prefabricated house?

There are several options for buying a prefab home or ADU prefab, depending on your needs and preferences. Here are some common options:
1. Manufacturers: There are many factories that specialize in designing and building prefab homes. These manufacturers often have a wide range of designs and customization options available. They can build the home in a factory and transport it to your site.
2. Online marketplaces: There are several online marketplaces where you can buy prefab homes, such as Amazon, eBay, and Houzz. These marketplaces often have a wide range of styles and designs available. Due diligence is on you. Always.
3. Contractors: Some contractors specialize in building prefab homes. They can help you design and build a custom prefab home that meets your needs and budget. No contractor can do it all. They typically specialize in one type – be it panelized or traditional stick built.
4. Vendors of modular homes: they contract factories to build units to their specifications. Often these plans are developed in cooperation with world famous architects or design studios.
Many options exist for buying a prefab home. The best option for you will depend on your specific needs and preferences. It’s SUPER important to do your research and compare different options before making a decision. Most companies do have websites with beautiful pictures of built homes. It is your responsibility to do due diligence so that you won’t be caught in the middle of the project by having to find a crane, local contractor  or permit expeditor.
We are here to help you build the right ADU for you

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