ADU Design in New Home Plans

If you’re considering adding an accessory dwelling unit (ADU) to your house plans or as an option, you’re in good company. Whatever you prefer to call it – a granny flat, in-law suite, or multigenerational suite – ADU construction is growing 9% per year, and more architects and builders are including them in their new home plans. Of all single-family homes designed in 2021, roughly 15% included a lock-off unit or rental room or suite, according to Mikaela Arroyo, director at John Burns Real Estate Consulting. Of course, not all homes designed with an ADU option will be constructed with one, but the figure indicates an increasing desire for home plans that accommodate evolving households.

Multigenerational living is a major contributor to this growing trend. The number of families with more than one generation living under the same roof has increased fourfold since 1971. Financial concerns and caregiving responsibilities top the reasons why—rising inflation, housing affordability, and elder care costs will continue pushing households to seek home designs that accommodate an additional family member comfortably. An ADU also provides a more affordable rental option and a potential source of income for the homeowner.

In this blog, we’re talking about all things ADU, including the various types and the best way to include one in a new home plan that appeals to a diverse buyer profile.

Table of Contents

Benefits of ADUs

A home plan with an ADU included or as an option can be a differentiator that sets your community apart from the neighboring competition. Here’s how:

Home Plan Diversity

Master plan developers continually seek diverse product offerings to appeal to more buyer profiles. Indeed, they are also paying attention to the growing multi-generational living trend that is taking place across many areas of the country. Including an ADU option in your house plan portfolio shows that you are also poised to address this growing buyer segment.

Missing Middle Housing Alternative

Homes with ADUs could help improve a city’s missing middle housing shortage, albeit in a small way. When the ADU is used as a lock-off unit, it potentially creates a source of rental housing that may be more affordable than alternatives in the area.

Homeowner Financial Benefits

Financial benefits to the homeowner are also unique selling points of an ADU home plan. Elder care costs are ballooning across the country, with the financial burden often falling on the aging parent’s middle-aged children. An ADU could potentially save the homeowner thousands of dollars per month in elder care costs.

An ADU’s flexibility lends itself to a variety of purposes beyond multigenerational housing. It can transition to a rental property, either as a long-term rental or short-term rental (Airbnb, VRBO, etc.). Either rental situation can provide another revenue stream or an opportunity to pay off the home’s mortgage early to save thousands of dollars in mortgage interest.

Types of ADUs

There are a few different types of ADUs, and it’s important to understand the characteristics of each to determine the best fit for your home plans. It is also helpful for your sales and marketing teams to be familiar with these accessory dwelling unit ideas as they develop messaging and sales communication in their outreach efforts.

Internal ADU

This type of ADU is completely integrated into the primary residence layout. In other words, it’s designed to occupy the best location to facilitate optimal flow. An internal ADU is a “home within a home,” and because it’s fully contained within the primary structure, it isn’t visible from the exterior.

Attached and internal ADUs maintain an uncluttered streetscape that is often preferable to master plan developers and municipal design review boards.  

This home plan features a 2 bedroom, 698 SF accessory dwelling unit on the main level. Design by KGA Studio Architects.
This 2,278 SF home plan features an internal ADU on the main level with the primary residence upstairs. Builder: Green Valley Homes.

Attached ADU

Not to be confused with an internal ADU, an attached type refers to an addition that is designed outside of the home’s original floor plan. Attic and basement conversions fall under this umbrella, as do ADU designs that are not integrated in a home’s layout.

Detached ADU

This type of dwelling is a separate structure that is independent of the primary or larger home. Examples include a detached garage conversion or addition, casitas, cottages, or a unit above a detached garage. 

This custom carriage house with an apartment on the upper level is an example of a detached ADU.

How to Design an Internal ADU

Offer your buyers a floor plan that stands out from the competition with a thoughtfully designed internal ADU (home within a home) that accommodates extended visits, live-in relatives, or tenants.

Design for Flexibility

People transition into different life stages and their needs evolve. Add flexibility to your ADU layouts just as you would in your standard house plans. A homeowner may welcome an adult child back home to live temporarily one year, then move in a parent with health concerns a few years down the road. Or they may decide to rent the ADU as an apartment to generate passive income. Or they may even choose to downsize by living in the ADU themselves and renting out the primary residence!

An ADU design should be able to accommodate these changing needs, like an open floor plan and a well-sized bedroom and bathroom. Integrating universal design elements is also an important consideration, like wider doorways, non-slip flooring, and grab bars in the bathroom.

Add Volume in Creative Ways

The majority of ADUs fall within the range of 600 to 800 square feet (although some are as large as 1,200 square feet or more, depending on municipality allowances). In Denver, ADU size limits are tiered based on the property’s lot size—650, 850, or up to 1,000 square feet.

But in general, most ADUs are on the smaller side and, as such, require careful design consideration. It’s easy for this type of dwelling to feel small, dark, and cramped, so pay attention to its location within the home or on the lot, its orientation, and access to natural light.

Ceiling height is also important. The taller the better to add some vertical volume to offset the smaller layout and room sizes. Keep sightlines open with pony walls and a window to enjoy property views if possible.

Allow for Privacy

A private entrance with a locking door is a must for an ADU that will become a rental property. It’s also good practice to include one even if the unit will be used for family and other long-term visitors.

Window size and placement should also be addressed. If the ADU is a carriage unit over a garage or studio, windows should face inward toward the property to eliminate any privacy concerns from next-door neighbors.

New home plan with an internal accessory dwelling unit (ADU). Designed by KGA.
Designed for narrow lots, at 30′ wide and 3,383 total square feet, this home plan features a 1-bedroom internal ADU located over the garage at the rear of the home.

Provide Storage

Make everyday living comfortable and uncluttered with plenty of creative storage opportunities throughout the ADU. Niches, alcoves, and other recess storage spaces should be plentiful. Storage solutions that work well in ADUs include built-ins in the living area, recessed storage built into the tilework in the bathroom, and an under stair storage closet in a bottom-level unit.

A storage option outside the ADU is also helpful for larger and unused belongings. Consider walling off a small section of the garage as an additional storage area with an exterior door that locks for privacy.


What is an accessory dwelling unit?

An accessory dwelling unit or ADU is a self-contained second home on a single-family residential property. An ADU includes a sleeping area, living area, kitchen, and bathroom. Many but not all include a private entrance. ADUs can be designed internally in the home, as an add-on to the main home (basement or attic conversion), or a detached structure independent of the main house. ADUs share the same energy and water connections as the main house.

How big can an accessory dwelling unit be?

ADUs range in size, but 600 to 800 square feet is the most common. Many municipalities have specific size limits on ADUs depending on the size of the property. Check your local zoning and building codes to determine if the property in question is zoned for ADUs and for specific building restrictions.

Do ADUs add property value?

Yes! In a 2021 study by Porch, homes with accessory dwelling units sold for 35% higher than average nationally. In Denver, ADU properties sold 31% higher than average.

Are ADUs allowed in Denver?

Denver does allow ADU construction, however not all properties are zoned for them. Denver published its ADU regulations and zoning information on the city website and as a PDF download.

Does an ADU count as square footage?

Yes, the addition of an ADU increases the amount of livable square footage on a property. How an ADU is appraised, however, may vary from one municipality to another. The type of ADU could also affect its appraisal – a square footage review and comps may differ if the ADU is detached or internal/attached.

The accessory dwelling unit is an up-and-coming product segment, and we anticipate interest to only increase as issues of housing affordability and elder care continue constraining U.S. households. Contact us below to discuss how to incorporate an ADU option in your house plans.

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