Modular ADU: Make No Faux Pas in a Hurry

Modular ADU

What you need to know about modular accessory dwelling units

5min read
by Olesia Chikunova, ADU Specialist.

Modular ADUs are built in a factory and shipped to your site. This is the most frequently mentioned way of prefabricated construction. Generally, the homeowner will work with the modular company to select and often customize their unit.

A general contractor is then required for site, utilities, and concrete work in preparation for the installation of the modular unit. After the unit arrives, the general contractor will supervise connection of the utilities (hook ups) and pass the final inspection that allows occupancy.

This method simplifies the design process to choosing a company and then selecting a model and finish options. Modular construction generally takes the least time from conception to completion with a reliable level of quality control.

Why Modular ADU?

In theory, a modular ADU makes permitting  easier, but because it is a relatively new and not yet widely implemented method, some jurisdictions may have additional questions that require additional effort from your permit expediter.

Additionally, site-specific conditions on your property, such as access to utilities and where your main sewer line is or whether a prefab home can be delivered to the site, may impact your decisions on whether a modular ADU will save time and money.

Modular building is definitely a trend. You hear about startups getting funded. The newspapers broadcast the benefits of factory built construction.

One search yields 1,130,000 results.

Tiny home ADU

Biggest surprise with modular ADU

What nobody tells you is that it still costs the same as a custom build house and often takes as long while taking all the benefits of customization out. You are limited by the choice of floor plans and finishes. You have limited choice of modular builders familiar with your jurisdiction. As a result instead of selecting out of multitude of options you are down to selecting out of 2 max. And if something goes sideways, you are stuck. 

TLDR, go modular if:

  1. You like the look of their display unit and have a way to guarantee your unit will look the same.
  2. You have the spot in your backyard that this unit fits perfectly.
  3. The floor plan works for your needs.
  4. It is really important for you NOT to see framers on site.
  5. Your modular builder has experience with your city and can prove it by showing  final inspection records of previous projects.
  6. You have seen the model in real life.

Can you imagine buyer's remorse on a $200k purchase? 

One cannot return a house that the crane had just deposited in your backyard.

Fun fact: most of modular builders expect full payment ( or at least, money in escrow) before the unit gets shipped from the factory.

Why go modular?

It is the right thing to do.

You want to do things right because you heard it was a more efficient way to build. It is, to a point. The factories indeed have a crew that builds the same model day in, day out.  You might be surprised but these factories are also experiencing the labor shortage that is a huge issue in the industry. You might want to inquire how long is their backlog.

Have you heard the phrase – built in controlled environment? They do build them under the roof, not in the rain or under the snowfall. Side note: it often rains in California when you are just about to start foundation or cover the roof. Ask anyone who had recently built anything. But seriously speaking, how much does it rain in California?

So how important is the benefit of factory built construction in this sunny State?

container home
Image courtesy of Steelblox

Modular construction is the most environmentally friendly way to build. 

What exactly is green building?

Local materials. Sustainably sourced. Efficient insulation and windows that allow for low utility bills.

Do you know that LEED (Leadership in Energy & Environmental Design) system that verifies a building is a best-in-class sustainable building, will not count the materials if they are shipped to your location from farther than 300 miles? I have met several homeowners who have ordered a modular unit and had no idea it was coming from a different country.

Where is your unit coming from? Is it traveling across several states? Where did the windows travel from to get to the factory – are they traveling back now already installed?

When your modular builder talks about efficiency – can it be confirmed by data? If they are stating the same R value, that a building code required, then where is the extra effort? Have they done a blow door test to prove the insulation is done well?

There are other ways to go green – you could use prefabricate panels or precut home kits to the same result – with more control of quality on site, where you actually see what is being done, and can interfere any time if it is not going according to plan.

Is there a distinction between a modular factory and manufactured home factory?

Actually, not so much. They often produce both manufactured and modular houses.

For the record, there are 158 manufactured home factories in the states as of 2017 according to the Manufactured Housing Institute. Eight in California. 

Modular Builder for Your ADU

How to pick your modular builder? When should one go modular?

Note: Everything written here is about construction of ADUs in California. It does not relate to multi family buildings or multi-unit residential developments.

Quality of modular build

How do you determine that the quality of the building is there?  How do you know it will survive years of use? Have you checked the factory, and how  long they are in this business, and what their rating is? Have they just opened shop?

Modular Display Unit

When a modular builder is showing you their display unit – do you think that they are showing it in the state it arrived from the factory or patched up for transportation cracks and other defects and polished to sell – pretty much as any house before it goes on the market?

Speed of modular construction

No doubt, you will not see framers on site. The unit will arrive on a bed of a truck, then the crane will efficiently land it in your backyard. Will you skip all construction activities? Not exactly.

The foundation needs to be poured. May be some demo required before that. The utilities need to be taken care of: plumber, electrician, HVAC. Fire sprinkler company if your main house has them, or if your property is located in the fire hazard zone. A deck to step out of the new unit will need to be built on site.

Thus, you are not escaping the construction activity altogether. You still have some three weeks of work before and 2 weeks after. At least. If we lived in a perfect world. 

Modular building is a new trend.

You are not just blindly following a trend, are you? You should be thinking about your property value in the long term – will it benefit more for a 400sf rectangular modular unit or a 600 custom shape site built? You are making the trade off between time of construction on site and customization. 

Modular accessory unit

Learn from our school of hard knocks

Oh, you already picked a modular builder , and now want to make sure you are not making a mistake…

What questions have you asked your modular vendor?

Have you seen their finished project in situ? It cannot be one display unit.

Have you asked them how they do quality control? Red flag: video inspections.

Have you talked to their customers? Have you asked them what demanded their attention the first months they were using the unit? 

How long did it take from the day the unit arrived on a truck to the day they were able to use the unit? 

How long did it take to permit? What were the comments the building inspector made?


Mostly importantly, have you asked them if they would do it all over again with the same vendor?  If you are granted one question only, this is the one I would use. And if they say yes, what would they do differently?


How experienced is their executive team in construction? They need to have someone there who had build homes from scratch before and was responsible for the result – from day one to final inspection.


Do not compromise on your modular building. Do not compromise on your ADU.


Go modular if:

You like the look of their display unit and have a way to guarantee your unit will look the same.

You have the spot in your backyard that this unit fits perfectly.

The floor plan works for your needs.*

It is really important for you NOT to see framers on site.

Your modular builder has experience with your city.


*If you are building it for an elderly relative, please run the design by their occupational therapist or by someone certified in Home Modifications. Not all units that claim being age friendly, actually are. In the same way not all products saying they are ADA compliant, are suitable for everyone. Big red flag? A porch with five steps up.


dreamy modular unit

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