Attached Vs. Detached ADUS spreadsheet

You can add significant value to your property, earn a consistent rental income, or provide a home to an aging parent or relative. How? By constructing an accessory dwelling unit or ADU on your lot.

An ADU is simply a smaller, secondary home that is built on the same plot of land as the primary residence.

ADUs can be of various shapes and sizes. Depending on the reason why you’re building it, you will have to choose the type of ADU that best suits your needs and preferences.

Practically any separate living area on your property that has its own entrance, kitchen, bathroom, and bedroom, can be considered an ADU. Naturally, this allows for many different types of structures.

Available space, local zoning laws, and intended use are just some of the things you will have to consider when determining the details for the ADU you want to build on your property. (Here’s more on how to find out what your local laws are.) But your first choice is between two categories of ADU.

These are known as attached and detached ADUs – the two main types of accessory dwelling units that you can construct. Apart from these, there are various subcategories, including garage conversion ADUs, basement ADUs, junior ADUs, and more. But every ADU can first be categorized as either an attached or detached ADU.

The primary difference between attached and detached ADUs is that, despite being a separate living space, an attached ADU shares a wall with the primary residence and is, therefore, attached to it. A detached ADU, on the other hand, is separated from the primary residence by a piece of open land.

An ADU constructed in the basement or the attic of the primary residence will be considered an attached ADU. And the amount of distance (known as setback) between a detached ADU and the primary residence may vary depending on the local laws and the available space.  

In other words, an attached ADU might be a part of the primary residence that contains all the facilities of a separate living space. However, a detached ADU is a separate, smaller house built on the same plot of land as the primary residence. For this reason, detached ADUs are also known as backyard cottages.

Differences Between Attached & Detached ADUs

To help you decide on the type of ADU that will be best suited to your needs, we have compiled a list of the major ways in which attached and detached ADUs differ from each other. Do keep in mind, however, that this is not a comprehensive list.

The exact type of ADU (in terms of size and design) that you can build will depend, largely, on your local zoning laws, your lot, and your budget. So, you will need to do some of your own research before making a decision about your preferred type of ADU.

That said, there are some differences between attached and detached ADUs that are more universal in nature. These are as follows:

1. Maximum Size

When choosing between an attached or a detached ADU, the size of the dwelling is one of the major differences that you will have to keep in mind. This is because there are different restrictions that govern the maximum size of an attached or detached ADU.

For instance, in the state of California, the ADU Ordinance dictates the amount of square footage you are allowed to add to your existing home when you build different types of ADUs.

In most cities and counties of California, you cannot build an attached ADU that is larger than half the size of your primary residence. This means that if you live in a 1,500-square-foot single-family home, then your attached ADU cannot be larger than 750 square feet.

The restrictions might be even more stringent in your area. For instance, in the city of Sonoma, an attached ADU can be no bigger than 30 percent of the existing living area of the primary residence.

A small gray detached ADU next to a larger gray house; size requirements for ADUs.
Different locations have different laws about the size of ADUs.

On the other hand, in most parts of California, a detached ADU can be as big as 1,200 square feet (as long as there is sufficient space for a dwelling of that size) regardless of the size of the primary residence.

While different jurisdictions within California have their own size-related regulations when it comes to ADU construction, all detached ADUs can be built up to 800 square feet, regardless of which part of the state you’re in.

Most jurisdictions also require that a detached ADU be less than 16 feet high, and that it must have rear and side setbacks of at least 4 feet each. On the other hand, an attached ADU can be as tall as the primary residence, and naturally, does not require any setbacks. An attached ADU might be a good idea if your lot is relatively small, as it will maximize lot coverage by eliminating the need for any setbacks.

This is important because the size of an ADU affects everything from the market price of the property and the potential rental income to the possible uses of the structure. Moreover, some of the fixed costs of construction remain the same, regardless of the size of the ADU. So the size of your ADU could have a significant impact on how much return on investment you can realistically expect.

2. Construction Costs

In terms of the cost of construction, there is a significant difference between attached and detached ADUs. In general, detached ADUs are much more expensive to build than the ones that are attached to the primary residence.

For instance, a detached, one-story ADU in an urban area in California might cost anywhere between $300K-400K. Since detached ADUs are usually built from scratch – with no pre-existing foundation or other support structure – they cost as much as a small, independent house.

On the other hand, building an attached ADU on the same plot of land will probably cost you between $200K-350K. Attached ADUs are cheaper because they share one or more walls with the primary residence. Often, they are built on an existing foundation.

Since they do not need to be constructed from scratch, they also require less by way of raw construction materials. Due to the reduced requirement of materials and labor, attached ADUs are the more affordable option.

A two-story addition being constructed on an existing home. Attached ADUs can be added on.
Adding on to a house will almost always be more affordable than building a second structure.

The cost of an attached ADU may fall further if it is located in a finished attic or basement of the primary residence. To outside observers, such an ADU may not look like a separate housing unit. However, if it has a kitchen, bathroom, bedroom, and a separate entrance, then it would meet the requirements for being legally recognized as an attached ADU.

(One way to create separate entryways for a basement or attic ADU is to have a common mudroom area that forks off into two separate entrances, one leading to the ADU and the other to the primary living area.)

3. Potential Rental Earnings

You will most probably earn a higher rental income from a detached ADU than an attached one. The reason for this is twofold.

  1. Detached ADUs are typically larger, since they are not limited to being only half the size of the primary residence. Renting out a larger living space automatically means that you can charge a higher amount in rent.
  2. A detached ADU offers much greater privacy to the tenant. You can create more separation between the detached ADU and the main house by using shrubs (or a fence) to cordon off the part of the yard where the detached ADU is located.

The larger living space and the added privacy ensure that homeowners can charge more for detached ADUs. For instance, in Portland, Oregon, a 750-square-foot attached ADU, located in the basement of a single-family home, will probably fetch you about $1,200/month in rent. On the other hand, a detached, two-bedroom ADU in the same location would command no less than $2,000/month in rent.

However, you need to remember that the actual amount of rent you receive is not your ROI. Building and maintaining a detached ADU involves various expenses that you wouldn’t need to worry about with an attached unit.

Apart from being more expensive to construct, a detached ADU also needs some heavy-duty appliances like a water heater and a furnace. Separate utility hookups and sewer systems also add to the cost of owning and operating a detached ADU.

After the upkeep costs and utility fees, you can expect to make a net income of about $850 per month from your two-bedroom detached ADU. The net monthly income from an attached ADU, if it is rented out, might be around $500. (These numbers are just examples—they will vary widely depending on the location, size, and design of your property.) (Further reading: How Much Can You Rent Out Your ADU For?)

4. Parking Requirements

The parking requirements for different types of ADUs can vary widely from one jurisdiction to another. There are no one-size-fits-all solutions to the parking issue, but we can offer some general guidelines and highlight recent trends in the regulations surrounding ADU parking. Still, at the end of the day, you will have to do your own research to find out everything you can about the parking requirements for ADUs in your area.  

For detached ADUs, the city of Menlo Park in California requires at least one off-street parking space. The parking space can either be covered or uncovered. It can be located at the front, rear, or side of the ADU. If a garage is converted into an ADU in a single-family district, then you need not construct a replacement parking space for the primary residence.

However, if the property is located in a historically or architecturally significant district, or if it is within half a mile of the nearest public transportation facility, then no off-street parking is required for a detached ADU.

In the case of attached ADUs, the parking requirements depend on whether the ADU is located in the interior of the primary residence (in a basement or attic) or if it is added to the home exterior. An exterior attached ADU needs to have one off-street parking space of its own, just like detached ADUs.

On the other hand, an interior ADU does not need to have any additional parking space on the lot. However, overnight on-street parking is not allowed in single-family zoning districts in Menlo Park, so the lack of a parking space might limit your ability to rent out the ADU. 

5. Changes in Property Valuation

Attached and detached ADUs will affect the value of your property in different ways. In general, a detached ADU will increase the market price of your property and add more value to it than an attached ADU will.

Hand pointing to a chart with different factors influencing property value: location, market, age, condition, improvements and neighborhood. Improvements is circled because an ADU is an improvement that increases property value.
Many factors influence property value. An ADU is a significant improvement that will almost always add value.

However, an attached ADU can make better use of the available space than a detached ADU, thus providing you with more square footage of living space on the same amount of land.

So, if you have a compact lot where every inch of space is at a premium, then an attached ADU might be a better way to add value to your property by making optimal use of all the available space. No land will be wasted in creating a separation between the primary residence and the ADU.

In most areas of Los Angeles, for example, the average cost per square foot for a single-family home is around $470. Therefore, a 1,000-square-foot detached ADU would increase the value of such a property by $470K, at a minimum.

Building a 1,000-square-foot ADU in Los Angeles would typically cost no more than $250,000 (including the cost of design, permitting, etc.), so the homeowner would make an approximate ROI of $220K, for overseeing less than one year of construction work on his or her property.

An attached ADU would also add a lot of value to the property, although it might be a little less than a backyard cottage. This is because potential buyers usually prefer a separate, private dwelling unit over an ADU that shares one or more walls with the primary residence.

Moreover, under most jurisdictions, an attached ADU cannot be larger than 50 percent of the size of the primary residence. This automatically limits the extent to which an attached ADU can add value to your property. The value of a property is usually determined on the basis of price per square foot. Since an attached ADU can only have 50 percent of the square footage of the main house, it can only increase the property value by approximately 50 percent.

Concluding Note

These are some of the major differences between attached and detached ADUs. To figure out which type of ADU is the most suitable for your property, you must consider several factors, including your budget, the size of your lot, the zoning regulations in your area, and your own long-term financial goals. 

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