Are you looking into adding an ADU to your property? Get off to a good start and familiarize yourself with the types of ADUs, different modes of construction, and other variables so you can build a solid foundation of knowledge for your project.

What Are ADUs?

ADU stands for accessory dwelling unit – a small living structure added to another building. You might have heard someone refer to ADUs with a different name before, such as accessory apartment or secondary suite.

As these names suggest, people often use ADUs as an extra living space. You can attach them to almost any part of your house or leave them unattached based on your needs. Because of its versatility, people can use their ADU for a multitude of purposes:

  • Provide a home for a loved one who needs assisted living
  • Prepare a lodging for guests
  • Building for a caregiver
  • Increase their home’s property value
  • Earn extra income from renting

Since ADUs serve many needs, they need to have versatile designs.

Types of ADUs

ADUs come in several different shapes and sizes to fit with any house. Both construction managers and homeowners need to know the common types of ADUs:

  • Detached New Construction ADU: a standalone unit often placed in a homeowner’s side or backyard as a new addition. They do not connect to the main house.
  • Attached ADU: extends from the primary residence, generally used as a garage or workshop.
  • Above Garage ADU: attached above the existing garage.
  • Garage Conversion ADU: existing garage changes function from a car storage space to a living area. It can be attached or detached.
  • Interior Lower-Level ADU: additional living space in the basement attached to the main residence via a new entrance, also known as a basement conversion ADU.
  • Interior Upper-Level ADU: additional living space in the attic, accessible via an outdoor stair and private door.

How Much Does an ADU Cost?

The cost of an ADU varies depending on the type, but many other factors can greatly affect the bottom line:

  • Construction
  • Design
  • Utility installation
  • Landscaping
  • Furniture
  • The builder
  • Site conditions
  • Costs of labor and materials in the area
  • Zoning laws

You might notice that size did not appear on that list. Size doesn’t contribute much to an ADU’s costs. The smallest unit can have more features and accessories than the largest unit. With that in mind, it makes sense to build an ADU with more extensive square footage.

If you’re looking for an average price, that number is quite random and likely won’t give you a reasonable estimate. The final number can fall between $80k for a modest detached ADU and $400k for a luxurious attached ADU. These structures have too many moving parts to predetermine the cost, so be prepared to bring on a consultant to help simplify things.

Additionally, since ADUs are a growing commodity and not yet mainstream, you might find it challenging to get a loan from the bank to finance the project. It will likely have to come out of your own pocket, which will put a dent in your retirement savings.

However, once you complete the ADU, you can rebuild your wealth by renting the unit out or making it an Airbnb. It’s also a much cheaper way to take care of loved ones who need assisting living than sending them to an expensive retirement community. You can naturally make your money back over time.

Building Your ADU

Before you decide where to build your ADU, you first must determine why you’re making it. The why will influence your decision to build on-site or somewhere else. Carefully consider the following factors:

  • The person/people you’re building for
  • The type of ADU that best suits their needs
  • The amount of property you have to work with
  • The property’s layout

The user might need special accommodations, like handicap accessibility. Some ADU types support that function better than others. If you have enough space, you should use it. If your property has a flexible layout, you can make all kinds of customizations.

On-Site vs. Modular: Which Is Better?

If you opt for a detached ADU, modular construction is the better choice. The attached ADUs are all built on-site as a necessity. But which building method will give you the most cost-effective results? Let’s weigh the pros and cons of each.

Modular Building

Modular building, also known as prefabricated or “prefab” building, applies only to detached ADUs. The builder constructs the unit in an off-site warehouse, then transports it to the site at a predetermined date. The only thing that requires on-site building is the foundation so that the completed unit can be easily lifted into place when the time comes.

Here are the main advantages of modular construction:

  • Fixed Costs: site evaluation, permit acquisitions, and total construction expenses fall under a single fixed cost.
  • Fixed Timeline: with no independent variables like weather, the timeline can remain undisturbed and under quality control.
  • Speed: building off-site and installing the unit is much quicker than traditional on-site construction.
  • Cheaper: since off-site construction is faster and more efficient, it’s naturally the more affordable option.

No matter the builder you decide to hire, they will have a much easier time completing your ADU within a secure warehouse than on your property. The construction and installation process will go smoother for everyone. You just have to set the foundation in place.

You should also know the potential downsides of modular construction:

  • Inaccessibility: not all properties can handle the delivery of a completed unit.
  • Fixed Plan: once the construction process begins, you can’t make any changes without throwing the timeline off course.
  • Limited Customization: it’s difficult to make customizations on a pre-planned building.
  • Lines of Communication: careful coordination is required between the on-site and off-site construction crews.

On-Site Building

As you might expect, the construction process of attached ADUs is a little more complicated, but this method comes with some appealing benefits:

  • Flexibility: you or the builder can make changes after construction begins.
  • Customization: on-site construction opens a world of design possibilities.
  • Convenience: most urban and suburban areas can’t fit a completed unit, but on-site construction offers an agreeable solution.
  • Renovation: you can fix/renovate parts of your existing home attached to the ADU.

Versatility is arguably an ADU’s most attractive trait, and on-site attached units embody that quality far better than off-site detached units. However, some negative attributes can offset that versatility:

  • Extra Fees: with every customization or new design comes additional fees from the architect or engineer.
  • Weather: the elements can potentially throw a huge wrench in your plans and even cause severe damage.
  • Safety Concerns: outdoor on-site construction always poses more safety risks than working in a controlled factory setting.
  • More Expensive: The above variables and an attached ADU’s design flexibility make on-site construction more expensive than modular construction.

To recap: modular construction is safer, more predictable, faster, and thus cheaper, but you’ll find it difficult to make any changes once the construction begins. On-site construction is more flexible and convenient for most homeowners, but it can also be unpredictable and more expensive.


Whichever method you choose, make sure you read up on your local zoning laws and acquire all the necessary permits before starting. To get the permits, you’ll need to submit the following materials:

  • All applications and forms requested by your municipality
  • Construction drawings/blueprints
  • Payment of the building permit fee

You should get a response within 10 business days if all the materials are in order. Once you get your building permit, you can begin your project. The municipality will occasionally check in on you throughout the construction process.

As for zoning laws, your area could also subject you to other regulations depending on your home’s location, size, and age. If you’re ever confused about these regulations, contact your municipality before making your next move.


ADUs are versatile units that you can build off-site or attach directly to your existing home. Off-site/modular construction is the safer, cheaper option, while on-site construction is more versatile and expensive.

Take your home and property’s layout into consideration when choosing a method, and make sure to fill out the necessary paperwork and research your local zoning regulations so you can get to building a functional and secure ADU.


In case you have architectural, structural, and MEP design requirements, or need a modular design for your home, feel free to contact us.  We provide you with the full permit set design + T24.