Planning for an ADU (Accessory Dwelling Unit)

ADU’s are becoming increasingly popular across the US. They are typically small, detached (or attached) living units subsidiary to a main home, that typically include at least 1 bedroom/ studio, a bathroom, and a kitchen. They are often designed to be used for family members, visitors, just extra space, or to be rented out long term or short term (airbnb).

Here we will go over some of the major, common regulatory considerations when planning for an ADU. Every locality will have widely different rules regarding generally whether or not they are allowed, and the specific zoning regulations regarding things like setbacks, height, max area, etc.

Is an ADU allowed on my lot?

This will be the first question you want to ask. You will typically want to dig into your localities’ zoning code if available, often found via google search. If you can’t find it online, then reach out to your local zoning/ planning/ building departments. This is often at a city/town level, but sometimes at county level.

Depending on what is commonly referred to as your “zone district” and possibly other factors, you should be able to determine if an ADU is generally allowed on your site. The local regulations might not use the term ADU. They might use a similar nomenclature, or may just refer to # of dwelling units allowed, and # of structures allowed on a site.

Analyze physical constraints on my lot

Once you determine that an ADU is generally allowed, you can then take a deeper analysis to the constraints that an ADU may need to fit in on your site.

Common constraints may include:

  • Setbacks- There are often front, side, and rear setbacks to be considered. There may also be a setback. distance between structures that must be respected for an ADU. Your local code may simply have you use the overall/ same setbacks as the main house.
  • Max lot coverage- Might be a percentage of the lot that can have structures on it. For instance if your lot coverage is 50% max, your lot is 5000 sf, and your home has a 2000 sf footprint, then you’d only have 500 sf left over for an ADU footprint.
  • Max Height, Max Stories, and Bulk planes. Bulk planes are imaginary boundaries sometimes used in zoning codes to make sure your structure slopes back from the property line as it gets taller.
  • Parking Requirements- Adding an ADU may mean needing to have a dedicated parking space for it.
  • Minimum lot sizes
  • Max ADU size- A basic square footage maximum that an ADU may be
  • Location on site- For example, it has to be behind the main house, or within the rear 35% of the lot

Other regulatory considerations

Most of the prior regulations would typically fall within a zoning/ planning type of departments purview. But, there are sometimes other regulatory bodies that might have a say into whether or not an ADU can be allowed. Some of these may include:

  • Wastewater- If you are on septic, you’ll surely have to prove that your septic system can handle an additional dwelling unit or upgrade if needed.
  • Water- Water departments might require a separate tap or some paperwork to process an ADU addition
  • Electric- Electric companies may require a new meter and service added for the separate ADU

Renting

If you are looking to build an ADU for renting purposes you’ll also want to look into the local laws for this. Most jurisdictions would allow long term rentals. Many jurisdictions have restrictions on short term rentals like airbnb. Denver, for instance, only allows short term rentals on properties that you maintain your primary residence on. So, in Denver, if you reside full time in the main house, and the ADU is on the same property, it can be rented short term.

Overview

As mentioned earlier, this is not an exhaustive list, and every locality has widely varying regulations. These are some of the more common regulatory considerations we come across. In addition to these regulations, you’ll also surely have to submit plans through a building review process to determine compliance with building codes and make sure it is designed structurally sound, usually with an architect and structural engineer. In addition to the regulations, there are also the financial and practical aspects of every project to consider as well.

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