When considering which type of attached dwelling unit to build on your property, you have options. One of the first decisions you need to make is which category to build: attached versus detached. Whether you opt for an attached ADU or a detached ADU depends on factors like desired size, budget, privacy and parking requirements, and expected return on investment. Some of these factors, however, will be determined by forces beyond your control, such as ADU ordinances in your property’s jurisdiction. Being armed with this knowledge should help inform your decision before you fall in love with a direction that may not be right – or possible.
Attached Versus Detached By Definition
Simplified, attached ADUs are those that share a wall with the primary dwelling and so are literally attached to it, while detached ADUs are completely detached from the primary dwelling by a piece of open land. The commonalities between the two are: 1) Both are considered new construction projects, 2) Both have minimum setback requirements of four feet from the side and rear property lines, 3) Both require a separate entrance, kitchen, bed and bath, and 4) Both have a height limitation of 16 feet under California law (although local jurisdictions may vary, as long as they do not impose a limitation below 16 feet).
What Difference Does It Make?
Size.
The number one reason most homeowners choose to go with an attached ADU is space. If you have a small yard, you may be able to create a larger ADU unit when it is attached to your primary dwelling. Keep in mind the four foot set back, which makes your available yard space even smaller. Attached ADUs are not options for multi-family properties, however, unless local jurisdictions choose to side-step this restriction (which Oakland has done).
Single family attached versus detached ADUs also have different maximum square footage restrictions. The downside of an attached ADU is a restriction on square footage to 50% of the primary dwelling floor plan, although the local jurisdiction may have a cap of 800 square feet. As an example, if your primary dwelling’s total floor area is 2,000 square feet, you may be able to build an attached ADU as large as 1,000 square feet.
An actual example will help illustrate where size has come into play in a significant way. An ADU project in Oakland’s Rockridge District could have been as small as 400 square feet due to a small backyard in which to build a detached ADU. Because of the decision to build an attached ADU, the unit is a noticeably larger 744 square feet – almost double the square footage.
In general, local jurisdictions can limit either attached ADUs or detached ADUs to 850 square feet for a one-bedroom ADU and 1,000 square feet for a two-bedroom ADU. Typically, if a local jurisdiction can cap the square footage, it will (most cities and counties do not want large ADUs). The good news is that local jurisdictions may offer more, but not less.
Your available space also comes into play when it comes to parking requirements, so do your homework and be sure to check with your local jurisdiction to make sure you will not be held to any parking requirements you cannot meet.
Privacy.
When considering an attached versus detached family home in such close proximity to a primary dwelling, another factor you need to consider is privacy (or lack there-of). In an attached ADU, you will be sharing at least one wall with your tenant/s, in-laws, or whoever will be living in your unit.
In the above example in Oakland, the design has taken the required sound transmission and fire containment mitigation into consideration. What could be a downside of the loss of windows on the attached wall has been overcome with the addition of skylights, as light is critical when there are little to no setbacks from the primary structure.
Detached ADUs, also known as backyard cottages, offer the advantage of separation, including the option of adding landscaping that may serve as noise detractors. Privacy may also be an incentive for what your tenants are willing to pay in rent.
Economics.
Attached ADUs will usually be more affordable to build; however, the cost-savings may be outweighed in the long run with the additional space – and potential increase in rental income and property value – that a detached ADU may provide. In most cases, if you have the space, it is likely a better investment to go with a detached ADU.
Still, there is no one right answer; and before you make the decision about which type of ADU is right for your particular situation, ask yourself the following questions:
- what is my available space.
- what are zoning laws that may impact my decision, and
- what is my intended use?
At Adapt Dwellings, the Bay Area ADU experts, we help you answer these questions. We guide you in moving confidently through the ADU process by leveraging our industry experience with the expertise we have navigating local planning and building organizations. Our goal is to remove any doubt that the ADU you ultimately choose to build can become a reality.
