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How to Setup Your Primary Home to Convert to an ADU Later

How to Set-Up Your Primary Home to Convert to an ADU Later

Hey everyone, today I want to talk about getting ready to build on a piece of property and how you can build an ADU first, and then go back and build your primary residence later. This pertains to the San Diego County area. To begin, you will build a primary residence first, but you must build so that it will meet the San Diego County’s requirements for an ADU in the future. This allows you to set your property up to where you can build a primary residence later without having to make modifications to the ADU, which is built first and considered the original primary residence.

Maximum ADU Square Footage Requirements for San Diego County

The first decision to consider is the actual size. The maximum size for a detached ADU in San Diego County is 1,200 square feet. That’s the living area of the ADU, porches and garage space can be added on, and they do not count toward the 1,200 square feet.

Other cities and locations may allow different livable sizes and garage sizes, such as the city of Poway which allows up to 1,500 square feet of livable area, but keep in mind, there may be HOAs that you must consider as well. So, consider what the maximum size possible is to build a detached ADU within the city, or building department, that governs the property you plan to build on. Checking the maximum size ADU that you can build is step one, which will prevent you from having to change the size of the ADU whenever you build the primary residence later. The driveway setup is the next step to consider.

Design the Driveway to Merge the Primary Residence & Accessory Dwelling Unit

Ask the question, “How is my property going to lay out?” Look at where you will want the driveway to go in for the primary residence and to the ADU. Lay that out so the future driveway tie-in looks intentional when that primary residence is built. They may have two separate entrances, but make sure to think through the plan of what the end project is going to look like with both homes on the property.

Decide on the Septic System Design

If the property is not connected to a city sewer system, the third step to plan out is the septic system. The septic system can be designed a few different ways. The most economical option is to install all of the necessary leach lines that will support the future primary residence, as well as the ADU that you plan to build. The first primary residence, which is the ADU, will convert over to an ADU once the primary residence, second project, is built. One option is to create one leach area that can combine all leach lines for both structures. If the property allows for a gravity flow system, you can have one septic tank for the primary residence and a separate septic tank for the ADU. Maybe the tank for the ADU is only 1,000 gallons and the tank for the house is 1,500 gallons. The separate affluent will flow down their respective tight lines that come out of the tanks and into one main line before going out to the same leach field. 

Remember that the first primary residence will turn into the ADU, and the septic for that structure will have a Y in the tightline, which allows for future tie-in from the tank from the future house. Another option works out to get a large enough tank to receive both houses depending on the size of the future primary residence. In this scenario, make sure that an inlet on one side of the tank is free to receive the future house waste line. This doesn’t occur often because the size of the primary residence is usually too large and requires a bigger tank.

Complex Septic Systems

Other more complex systems exist, known as an ATU system (Aerobic Treatment Units.) These involve pumping. If the elevation for one structure that sits lower than the leach field, then pumps and surge tanks are also necessary for traditional systems. Some of our other videos cover septic tanks and septic systems in more detail but designing the septic system for both units is important and if possible, install the leach field for both units at the same time. That process begins with the septic layout.

Set-Up Your Septic System for the Future Primary Residence

When you submit the septic layout, you want to show a proposed future primary residence with an ADU conversion to ensure that the health department is aware of the intent. Identifying the future intentions will prevent the septic layout from being reworked later as well as approve that future primary residence ahead of time. The layout will include the space for the primary system and the reserve area.

When Are Your Permit Fees Waived?

Another thing to look at is the permit fees. The impact fees for the ADU are often waived when building. That is true, however, it is only true when building the ADU after the primary residence exists on the property. These fees include your traffic impact fees and your park fees. If the primary residence already exists on property, then those fees would not be paid. Those fees are mandatory when building the smaller structure first, the one that will convert to the future ADU. These fees are paid because the home is currently considered the primary residence on the property until the larger home is built later.

Costs for Permit Fees in San Diego County

In this instance, the traffic impact fees can be anywhere from $3,000 to $8,000. Park fees can amount to $7,500 or so within the San Diego County area. You must pay school fees regardless, which are never waived. The Ramona area costs $3.48 a square foot for the living space. Poway currently ranges about $4.04 per square foot. Therefore, a 1,200 square foot home in Ramona would equal about $4,176 and the same ADU/home in Poway comes to roughly $4,848 in school fees. In San Diego County, your permit application fees will be reduced due to the small house size. When a primary residence already exists, the building permit fees are waived, in this case, the smaller structure still requires payment for the regular plan check fees.

How to Save Money on Sewer Utility Fees for a Home & Accessory Dwelling Unit (ADU)

Let’s review the utilities. Regarding the sewer, if you hook up to an actual city sewer, you must pay the initial sewer fee. That fee could range anywhere from $15,000 to $30,000 depending on the municipality. Typically, when you build an ADU second, the primary residence already paid for the ADU to get access to the city sewer, only the annual fee is required, which costs about $1,000 each year. This fee goes on your taxes. Sometimes the city may send a bill for the annual fee, but it’s a lot cheaper to build the ADU second, essentially saving the $15,000 to $30,000. 

Building the ADU later allows you to tie into the existing trunk line for the sewer system from the primary residence. When building the ADU first, be sure to install a Y in the trunk line for an easy tie-in on the primary residence, just like the Y for the septic tie-in. Before completing this step, check with your municipality to make sure that they will not charge another ADU fee to tie back into the sewer when building the future primary home, due to the order of events. This construction order is backwards from the norm, be certain that the city will not charge a second sewer connection fee since the first structure will be converted into an ADU, and the second structure becomes the primary dwelling.

San Diego Gas & Electric Utilities

SDG&E needs to supply power to the property as well. Make sure that you’re properly setting up the most economical method to get power to the house, as well as the gas lines. When the closest point of the ADU and primary residence are a minimum 150 feet apart, each structure may have a separate meter installed. When both structures are closer than 150 feet apart, then a dual meter is required. A dual meter typically requires a 400-amp service with one meter going to the first building, and the other half of the panel is blanked off for the future primary residence. The empty half of the service panel gets a conduit sweep stubbed out of the foundation for future connection.

The gas meters also need consideration when natural gas from the city is available instead of needing propane. The location of the two gas meters needs to be determined as well as the requirements needed by SDG&E to tie them into each building. These are the considerations for electrical and gas utilities. The County and Cities typically only require a minimum 10-foot separation from either house, therefore it’s easy to set two tanks of any size, especially when renting out the ADU.

How to Save Money on Water Utilities for a Home & Accessory Dwelling Unit (ADU)

Water is another utility to check on when building the ADU as the first primary dwelling. The water meter fee ranges in the $15,000 to $30,000 depending on the municipality. Like the sewer process, check with the municipality to make sure that they will not charge a separate $15,000 to $30,000 fee to put the second structure on the property because you’re doing it backwards again.

To simplify the water service to the future structure, we usually install a T connection onto that main line from the existing meter. We tie the second structure into that T in the future, and then the owners receive one bill that covers both the house and ADU. A separate meter can be installed for each structure, but at $15,000 to $30,000 per meter, most owners elect to install one meter.

Reviewing the Factors When Building an ADU First

Consider these factors when building an ADU first. Setting up a property to build something smaller that fits your current budget is the way to do it. Check on the maximum size allowable to build. Check out the permits and fees that are involved with utilities and your building department, and don’t forget to give your attention to the septic when the sewer is unavailable. Make sure the driveway layout and utilities will tie in easily in the future, and research what those costs will entail.

Other Helpful Resources

I hope that was helpful. If you’d like more information on building a custom home, ADU, or remodeling your home, you can visit our website at www.tfgonline.com. You’ll find a ton of information on pricing and floor plans, as well as many other resources that we have available. Thank you for your time again and have a great rest of your day.

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