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Accessory Dwelling Units

Are you considering building a backyard home or converting an existing structure?  Here, we focus on issues related to creating a second dwelling that is subsidiary to your main house on your lot.

The technical term for a backyard home in many areas is “Accessory Dwelling Unit.”  In the simplest terms, an ADU is defined as any finished living space with a separate entrance that includes a kitchen, bathroom and bedroom.  They come in the form of:

  • Backyard homes (guest houses, in-law suites, granny flats)

  • Garage or barn conversions

  • Apartments above a garage

  • Attached apartment additions

  • Basement apartments

  • Attic apartments

How large can an ADU be?  That depends on the zoning regulations where you are building, and it can sometimes be a complicated question to answer.  It's best to call your local permit office to find out.  The good news is that most, if not all of the designs shown on the plans page usually work out fine, assuming an ADU is allowed on your lot. 

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Our Focus

Our main expertise is with detached backyard homes and guest houses.  These add loads of flexibility to your home.  In addition to the obvious uses, they evolve over time to be a home for an adult child, a home office, or the property owners may move there as empty nesters so they can age in place and rent out the main house for retirement income.  Don’t laugh, but a second home may save a marriage if a couple is generally happy but would benefit from different living styles (neat vs. messy, sparse vs. cluttered, grandiose vs. intimate, and so on).

The architecture of an ADU is relatively simple: it’s basically just a miniature house and has to meet all the other applicable building code requirements for structural design, insulation, sprinklers, mechanical electrical and plumbing, etc.  A good architect can make sure it looks integrated with the main house, and we always look for opportunities to make the building suitable for aging in place without major renovations in the future.

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The more challenging aspect of building an ADU is often the site planning.  Zoning codes vary widely, so you may or may not be able to get an ADU permitted, even if you technically have room for one.  A few things that most zoning codes regulate: 

  • Are ADUs allowed in the that zoning district

  • How much floor area (all floors) is allowed

  • How big does the lot need to be to allow an ADU

  • Setbacks from the property line and other structures

  • How much impervious lot coverage is allowed on the lot (includes the main house, driveway, patios, sheds, pools, PLUS everything associated with the ADU)

  • Usually the structure needs to be behind or next to the main house; it usually cannot be located in front of it

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And on top of that, there may be restrictions on how the property can be used: 

  • The two structures usually cannot be subdivided into two lots in the future

  • Often, the property owner must live onsite and is sometimes allowed to live in the ADU

  • Who can live in the ADU (anyone vs family only)

  • Whether the ADU be rented or generate income

  • Whether the ADU be used for commercial purposes, such as a home-based business

  • Look out for additional restrictions on the guest home’s location from wetlands, streams, tidal water, steep slopes, floodplain, poor soils, etc.

  • If the LOT is not served by public water and sewer, is a shared well allowed, and can a new or expanded septic system be built?


Yes, that's a lot to think about, but we have decades of experience navigating the ever-changing regulations!  A good first step is to get in touch with your local planning/zoning/permit office and have an initial conversation about what you have in mind to do and whether they think it will be allowed.  Why not bring a Compact Home Plans flyer or two to the meeting to use as examples of what you have in mind to build?

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If things go well, the next step is often to line up a local civil engineer to prepare a site plan that works with the house plan(s) you have in mind.  If you are located in Maryland or Delaware, that could be Steve here at Compact Home Plans.  You might also ask Steve to prepare the architectural plans, or there are no hard feelings if you decide to hire another architect if have particular needs.  We can suggest several that we work with regularly.

Believe it or not, every single plan posted on this website can be permitted under certain circumstance within my home county, either as an ADU or as a primary dwelling.  I’m happy to help you look into what you can build on your lot, whether it’s in near my home base or not.  So have fun browsing the Plans page and happy house daydreaming!

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