An accessory dwelling unit (ADU) is an independent residential area located on your property but is separate from the principal single-family home. The accessory apartment, aka in-law suite, aka granny flat, or mother-in-law apartment, can be attached, internal, or stand-alone in the case of a detached accessory dwelling unit.  

This guide will provide you an overview of the detached accessory dwelling units (DADUs) such as a guest house or backyard cottage that’s structurally away from the primary residence.

And, here’s a point to ponder: a DADU increases your property value if constructed well. While it’s hard to pinpoint how much exact resale value it adds to your property, there are studies that show that ADUs and DADUs generally contribute to about 25% to 34% of your property’s assessed market value.

What constitutes an accessory dwelling unit?

Accessory dwelling units are self-contained living spaces that are built onto your property. Which type of ADU it is, will depend on its location relative to the primary dwelling unit.

Accessory dwelling units can be classified into three categories: 

  • Interior ADU – Conversion of existing space within the principal structure
  • Attached ADU – Adding living spaces to the primary dwelling
  • Detached ADU – Additional unit that’s structurally separate from the primary housing unit

If you’re aiming at an interior attached ADU, you can convert your basement, attic, or garage or add a living space to your home. All you need to do is simply give it its own private entrance, egress window, and a small kitchen, and your ADU is set. And, yes, you will have to cater for off-street parking space too. 

But if you specifically want to add a detached accessory dwelling unit, you can do so on to the back or side of your home. 

Keep in mind that these tiny houses, with foundations, are legal constructions only if they have been established through a permit process. 

Features of ADUs and DADUs 

  • They are significantly smaller than the primary dwelling unit.
  • They are one of two units on a single-family residential lot.
  • The units are owned by a single homeowner.

Can I build an accessory dwelling unit?

Whether it’s internal, attached, or detached, ADUs are gaining immense popularity. So much so that, many cities and states support ADUs in their plans and adopt specific zoning district regulations — permitting ADUs in low-density residential areas or single-family neighborhoods.

However, there should be sufficient room for a detached accessory dwelling unit on your property. For example, many states allow a DADU only if it’s at least 10 feet from the main home and 5 feet from any property lines. Also, you need to follow the permitted square footage limits, height restrictions, floor area ratios, and other metrics.

Keep in mind that the codes for building a DADU change from city to city. Therefore, you need the proper construction permit to build or an alteration permit to make the dwelling unit legal.

There are several reasons why ADUs are being promoted. They can increase housing affordability, for the homeowner as well as for tenants. That’s because they cost much less than buying a new single-family home on a separate lot. And, they create a wider range of housing options and supply within the community.

Most importantly, ADUs enable the senior members of your family, the empty nesters, disabled family members, or the young adults to stay near you. They get to enjoy their freedom and yet are secured in the knowledge that help (and love) is close at hand.

Also worth noting is that smaller homes consume fewer resources to build. And, they require less energy to heat in winters and cool in the summer months. Well, we’ll talk more about the benefits of accessory dwelling units below. 

What’s the location of a detached accessory dwelling unit?

A detached accessory dwelling unit is a smaller, standalone house built behind or next to your primary home. Due to its location, it helps you maintain your privacy while enjoying the square footage in your main house.

It can be above or within a converted garage or at the rear as a backyard cottage, laneway house, alley house, or a carriage house. 

Benefits of detached accessory dwelling units 


By whichever name they go by, ADUs and DADUs offer a lot of benefits for homeowners, the local economy, and even the environment. They provide:

  • Chance to house grandparents and seniors nearby who can live independently and securely. It supports aging in place and multi-generational living.
  • Affordable alternatives to nursing homes and assisted-living facilities which according to the U.S. Department of Health and Human Services, cost $3,500 to $8,000 per month.
  • ADUs and ADAUs can get you a rental income of $750 to $1,000 per month on average. The rates depend on the size of the ADU, location, and other factors.
  • An opportunity to provide short-term rental to travelers and other vacation rental sites if your jurisdiction has conducive short-term rental laws. Keep in mind that you’ll need to pay all applicable lodging taxes.
  • Additional, low-cost living space for your older children.
  • Good space for a home office, a studio apartment, a pool house, or a home gym.
  • Private accommodations for visiting guests. 

Drawbacks of accessory dwelling units

Since we always talk about both sides of a coin, it’s fair that we evaluate the downsides to an in-law suite as well. So, here’s a look at some common arguments against ADUs and DADUs in general:

  • Significant upfront cost – An attached ADU will cost you upwards of $40,000. The price goes up significantly in the case of a detached ADU that can cost you more than $100,000 to build and outfit.
  • Potential covenants and sale restrictions – Since there are varying zoning codes regarding ADUs, it can get a tad complicated at the time of a house sale. For instance, some jurisdictions have restrictions that ADU-endowed properties must be owner-occupied. 
  • Higher property taxes – An ADU could mean a steep rise in your property’s assessed value depending on the ADU type, its amenities, the location, and other factors. Therefore, it could lead to a significant increase in your property taxes – potentially adding a high-three or low-four-figure sum to your annual property tax bill. 
  • Long construction and permitting process – Like any building construction and zoning codes, the permitting requirements and the process can be very complicated and long. 
  • Greater maintenance – An ADU means more square footage, which means more upkeep of the accessory structures and maintenance costs. After all, you’re responsible for keeping it in good shape. 
  • Potential for vandalism – Detached ADUs are easy targets for vandalism as they are secluded. As a solution, you might want to install motion-activated floodlights or external cameras for extra security. 

How much does an accessory dwelling unit cost?

Adding an ADU or a DADU is a major investment. While ADUs are less than 1,000 square feet, they still require all the basics of a home. You need to install a small kitchen, bathroom, living room, and a bedroom.

The average cost to build an attached ADU is about $75 per square foot. That translates to $37,500 for a 500-square-foot space and $75,000 for a 1,000-square-foot ADU. DADUs cost is almost double! At $150 per square foot, a DADU costs $150,000 for a 1,000-square-foot unit.

Read more: Can a tenant hire a contractor? And who foots the bills?

Detached accessory dwelling unit: construction process

Building a DADU requires meticulous planning. Even when you hire a professional contractor, you must have an idea of the logical, step-by-step construction process. 

  1. Ensure that building a DADU is legal in your municipality.
  2. Determine whether your property is suitable for a DADU.
  3. Figure out how you intend to use your DADU.
  4. Hire the right professionals — general contractors, architects, or a designer — who specialize in ADU construction. 
  5. Thoroughly check their references.
  6. Secure your financing options. If you have good equity in your home, you may opt for a cash-out refinance or home equity loan. You can even consider a home renovation loan or an unsecured personal loan.
  7. Pull the necessary building permits from the municipal, county planning, or zoning department. 
  8. Remain compliant and vigilant throughout the construction process. 

Last words

Interestingly, the ADU and DADU movement is gaining momentum across the United States. Every year, thousands of homeowners evaluate the long-term benefits of accessory dwelling units and choose to build one on their property.

Apart from having a potential for a substantial rental income, it offers a flexible housing option for your aging parents or adult children. A win-win situation for everyone!

Read more: Practical Mother-In-Law Suite Designs

A Comprehensive Guide on Detached Accessory Dwelling Units was last modified: November 11th, 2022 by Ramona Sinha
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The information shared on the different units are useful and thanks for sharing this information with us.