ADU Homes: Prefab ADU, Modular ADU, Panelized ADU, Stick-Built ADU

An ADU, or an accessory dwelling unit (also called a granny flat or an in-law suite), is an additional house situated on the same property lot with a single-family home. Nowadays, these backyard cottages are becoming more and more popular in many cities of California.

In our article, we’ll look at the main ADU project types, their characteristics, pros and cons. We’ll tell you what a prefab ADU is and how it differs from the traditional accessory apartments.

ADU 101

Although ADUs can have different structural forms and floor plans, they’ve got plenty of things in common. It’s a completely unique type of housing, defined by several distinctive characteristics.

So, what makes a building an ADU? Let’s find out.

• An ADU is an accessory unit adjacent to the primary dwelling;
• It is much smaller than a typical house in the United States. Its size limit in California is 800 sq. ft.;
• A single-family can own two houses on the same land. One is the primary house, and the other is an ADU.

Benefits of ADU Homes

The demand for ADUs is growing rapidly. What are the reasons for it?

An ADU can serve multiple purposes, for instance:

• A profitable investment. Accessory dwelling units are full-fledged extra homes with everything necessary for a living. They have heating, plumbing, electricity, a kitchen sink, and so on. So, people often rent them out. They’re suitable for almost anybody — young, old, single, or married. Renting an ADU out can let you cover all its building costs in a very short time. Also, it can increase your property value. In case you sell your property in the future, an ADU will make it way more expensive;
• A living space for your aging family members. People call ADUs “granny flats” for a reason. If your parents or in-laws need constant care, an ADU can be super convenient for them because they’ll live maximally close to you but still have their own place;
• A guest house. For those who have many friends and often organize parties at home, an ADU is also a great variant. All the guests can comfortably stay there for a night;
• A mini-office. An accessory dwelling unit can provide a quiet space for online work. There, you’ll be able to concentrate on your tasks without your family members bothering you. So, the quality of your work will be much higher;
• An art studio or a personal gym. If you’ve got a hobby such as painting, making music, and so on, you can do this in your ADU, too, so no one will distract you. If you love fitness, an ADU is also a perfect place for it;
• A place to store your personal belongings, especially large ones.

So, as you can see, these tiny homes have many benefits. Even the smallest ADU can be suitable for lots of purposes. When choosing an ADU project type and its floor plan, you’ll need to take into account how exactly you’re going to use your new construction.

Now, let’s look at the most common ADU housing types.

Accessory Dwelling Units:The Most Popular Types

Accessory dwelling units can be very different: attached to the main home or detached from it, converted from an old garage or built from scratch, and so on.

The main ADU categories are the following:

• Attached. Such an ADU is a structure connected to the primary dwelling — for example, via a wall with a door. Oftentimes, access to utility connections is common, too. Attached ADUs are typically cheaper than detached ones, so many people choose them to save money. It’s a perfect variant for a large family or to keep belongings;
• Detached. Unlike the previous type, a detached ADU is a completely separate in-law suite. It may be pricey, but it’s ideal for renting out. That’s why the demand for these ADUs is super high;
• Converted. It’s an ADU built on the base of a pre-existing structure, such as a garage. ADUs of this type are rather inexpensive and quick to build. That’s why many people select it as an affordable housing option. Also, sometimes people build “carriage houses” – ADUs on the top of garages;
• Junior (JADU). It’s a miniature house, even smaller than a regular ADU. Its maximum size is only 500 sq. ft. A JADU can be a great living space for elderly relatives. In some places, zoning regulations allow homeowners to build it together with a regular one.

Also, there’s a large ADU category called prefab ADUs. We’ll talk about them further.

Prefab ADU. What Does it Mean?

A prefab ADU means “prefabricated” or “made in a factory” — at least partially. Builders simply transport parts of it to the client’s backyard and put them together. This makes such ADUs different from traditional or so-called “stick-built” ones. Compared to them, prefab housing units have plenty of advantages. They’re faster to construct, sometimes cheaper, and their building process causes minimal disruption. While a stick-built ADU requires from 7 to 10 months to be ready, a prefab in-law suite needs only several weeks.

The construction of stick-built ADUs, on the other hand, involves classic methods of framing. Typically, their primary material is wood (hence the name). Builders cut and assemble wooden pieces on the construction site. The whole process is slower, but gives the possibility for more customization of the floor plans and other things.

There are two categories of prefab ADUs. Let’s look at their main features:

1. Manufactured units. It’s the most widespread type of prefab ADU. It’s a complete construction that doesn’t need any assembling. Usually, workers just roll it to the backyard or lift it with a large crane. In the majority of manufactured ADUs, there are studios and one or sometimes two bedrooms. These buildings have standard plans which you can’t change. You can simply choose from a bunch of project variants that involve not only ADU floor plans but also materials, finishes, fixtures, and so on. These buildings often have beams and slanted roofs, so they resemble ordinary houses;
2. Modular (panelized) units. These ADUs consist of several prefabricated sections such as walls, ceilings, and floors. Workers deliver them to the property lot on a flatbed truck and put them together. After that, they add utility connections, roof tiles, and inner and outer finishes. This type of construction allows some degree of customization — although still much less than the stick-built one. Some companies even accept ADU floor plans from their clients. Usually, such houses have a box-like shape.

The difficulties of constructing prefab ADUs

At first glance, the construction of a prefab ADU seems pretty easy. But it has its own difficulties, too. First of all, an ADU is a secondary home by definition. So, it must fit into the constraints of the property lot, including its landscape, fence, and so on. For a prefabricated structure, it’s often much harder than for a traditional one. If you’re projecting a stick-built house, you can freely adjust its doors, windows, or floor plan to the terrain, while with a prefab construction, it’s impossible.

Prefab ADUs. Financing & Appraisal

As we’ve already mentioned, an ADU can significantly increase your property value. However, it depends on its type. The value of a stick-built ADU is usually way higher. Both home buyers and financial institutions view traditional ADUs as much better ones than their prefab counterparts. The estimate is especially high if the unit’s design matches the primary dwelling and the whole site. In most cases, it’s possible only if the ADU is stick-built because it’s much more customizable.

Prefab ADUs. Are they cheaper & faster to build?

Many homeowners who want an affordable ADU choose a prefab construction. But is it really cheaper? This may seem strange, but oftentimes, the costs are almost the same. If the ADU is small (for example, a JADU) and fits the site perfectly, it will cost significantly less than a stick-built one. But in case the construction is bigger, there’ll be almost no difference.

And what about the speed of the construction process? It may also surprise you, but the complete timeline — from obtaining the building permit and the project authorization to moving into the new home — is also quite similar. Although, the building process itself is, of course, way shorter. It takes only several weeks, while construction of a stick-built ADU lasts for more than half a year.

Frequently Asked Questions — FAQ

What are the drawbacks of an ADU?

Although ADUs have lots of advantages, they’ve also got several drawbacks. For example, they have very limited space — 800 sq. ft. or less. Because of this, living in such a house can feel cramped. Also, ADUs have rather strict building codes and zoning regulations. Before constructing it, you’ll need to ensure that your project meets all of them. Otherwise, the installation will be against the law.

What is the cheapest option for an ADU?

The cheapest ADU housing variant is a tiny prefab one — such as a junior ADU. Also, lower-cost variants include garage conversions and attached ADUs. However, we don’t recommend you to choose extremely cheap options – their quality may be rather poor.

What is the average cost for a prefab ADU in California?

The cost of prefab ADUs varies significantly, depending on such factors as its size, the materials’ quality, furnishing, and others. Usually, it’s within the range of $200,000-$400,000. But of course, there are also cheaper (even near $50,000) or more expensive variants.

What is the difference between a prefab and an ADU?

A classic (also called a stick-built) ADU is a housing unit made using the traditional construction method. On the other hand, a prefab ADU is at least partially ready when it arrives at the site. It can be manufactured or modular. The first type is a complete house, and the second one consists of several modules (for instance, floors and ceilings) that need assembling.

Can I put a prefab house in my backyard in California?

Of course. Prefab ADUs are very widespread in California nowadays. However, before ordering such a house, you’ll have to get all the necessary permits and make sure that this project meets the regulations of your site. The most optimal variant is to find a good construction company that will not only provide you with a high-quality, sustainable prefab ADU but also do all this bureaucratic work.