Getting Started with Your ADU Project
Building an accessory dwelling unit can be a complex process. Rost Architects recommends working with a qualified local architect who has experience navigating city requirements, approvals, and coordination with general contractors. Having a reliable team that has been through the process before can help avoid common pitfalls and keep a project on track from the start.
Rost Architects has been working in the ADU contractor space since 2017, designing and building a range of units for homeowners. Based on that experience, they have identified several key areas that property owners should understand before starting an ADU project.
Local ADU Guidelines: The city or county where the property is located will typically have a specific set of ADU guidelines. It is essential to understand these requirements before selecting or designing a unit. Key items to look for include setbacks, allowable square footage, height limits, and the required distance from the primary residence.
Setbacks: Setbacks define how far a unit must be placed from various features on the site, such as side yard or rear property lines. Most cities require 4-foot setbacks from rear and side yard property lines. Additionally, many cities require a minimum distance — typically 10 feet — between the detached ADU and the primary residence. Smaller lots may face challenges meeting these requirements, and an architect can help identify alternative solutions in those cases.
Maximum Floor Area: In many areas, the largest allowable backyard cottage or ADU is 1,000 to 1,200 square feet. Floor area is generally calculated from the outside face of the exterior wall, though mechanical spaces and covered patios are often excluded. Local requirements can vary, so it is important to verify this early in the process.
Height Limits: Cities often restrict ADU height to prevent two-story construction. In many areas, the limit is 16 feet. Confirming the height limit before finalizing a design helps ensure the unit will be permittable.
Site, Utilities, and Planning Considerations
Before designing a granny flat or in-law suite, it is important to understand the site boundaries. Some cities require a formal survey, while others allow access to a GIS system for boundary lines. However, GIS data is not always accurate, and Rost Architects recommends verifying boundary lines against the title report or any existing survey from a previous property sale. If documentation is unclear, having a proper survey completed is advisable, as setback violations discovered during construction can lead to significant problems.
Electrical: Many cities require a minimum of 100 amps of dedicated power to an ADU. Property owners should check their main panel to determine whether this capacity is available. If not, a panel upgrade may be necessary. Knowing in advance what systems — such as appliances, solar panels, or HVAC — will be used in the unit helps estimate power requirements more accurately. A qualified electrician and architect should assess whether upgrades are needed.
Plumbing: Understanding how water will be supplied to the new unit is critical. Factors such as main water line size, existing bathroom count, and the number of bathrooms planned for the ADU all affect whether the existing system can support the addition. Sewer connections are another important consideration — many cities require the ADU sewer to connect independently rather than through the primary residence's existing line, which can involve additional trenching and cost.
Gas: Many homeowners are opting not to install gas in their ADU, either for personal reasons or because local regulations are discouraging or restricting new gas connections. If no gas will be used, all appliances — including cooktops, water heaters, and HVAC — must be specified as electric. This decision should be made early in the design process.
How Rost Architects Approaches ADU Design
Rost Architects has developed a collection of ADU builder designs that property owners can select from and customize. Rather than starting from scratch, clients can choose a base design and then tailor specific features, materials, architectural elements, and layouts to suit their needs. This approach allows for a more personalized result without the full upfront cost of a completely custom design.
The firm works with individual owners in a boutique manner, handling projects with attention to detail. Their goal is to deliver units that are functional, well-designed, and suited to the needs of the household — whether that is a garage conversion, a Casita, a JADU, or a standalone backyard cottage builder project.
Proper planning is consistently emphasized by Rost Architects. Investing time and resources into thorough planning and design from the beginning helps avoid costly mistakes. While challenges will arise in any construction project, a qualified team is better positioned to manage those challenges and provide practical solutions as they come up.
