How Much ADU Can You Fit on a Small San Diego Lot?
The amount of space available for an ADU is not always obvious from looking at the yard. Property boundaries, existing structures, setbacks, fire codes, drainage, access, and utilities can all affect what is possible.
That is why ADU planning should begin with the property, not a predetermined floor plan or price per square foot.
A two-story ADU we designed and built in North Park shows how much these early investigations can matter. The homeowner wanted to replace an aging garage with a building that included a new garage, private office, indoor laundry, full bathroom, storage, and an independent one-bedroom ADU upstairs. Fitting that program onto a narrow urban lot required us to find and use every available foot.
Verify the Property Boundaries
A fence may appear to define a property, but it is not legal proof of the boundary.
On this property, the fence lines had likely shifted during the lot’s long history. A boundary survey established that the homeowner’s property extended approximately two feet farther than previously believed.
Two feet may not sound significant. On a constrained lot, it can materially change the building’s width, the interior layout, and whether the homeowner’s priorities will fit at all.
The existing garage introduced another opportunity. It had been built 2½ feet inside the actual property line. When this project was submitted, San Diego’s ADU regulations allowed the replacement structure to be built at a zero setback. Combining the land identified by the survey with the ability to eliminate the former setback gave us substantially more usable width.
Those regulations have since changed. The broader lesson remains relevant: establish the actual property boundaries and determine which rules apply before treating the visible yard as the buildable area.
Document the Existing Structure
The old garage was more than 100 years old, too small for a modern car, and no longer structurally sound. It had to be removed, but its history and legal status still mattered.
We documented the garage as an existing as-built component of the property. Under the regulations in effect at the time, the new ADU could be treated as attached, providing additional flexibility in setbacks and clearances.
Do not dismiss existing buildings simply because they will be demolished. Their footprint, location, use, and permitting history may affect the options available for a replacement structure.
Confirm Which Regulations Apply
ADU regulations can change while a project is being designed or reviewed.
Approximately ten days after we submitted this project, the setback provisions changed. The City initially determined that the application had missed the deadline because its intake invoice was issued after the change.
We had retained the City’s email acknowledging the original submission date. We used that documentation to challenge the determination and elevated the issue to the department supervisor.
Resolving the question added approximately two weeks to the permit process. It also preserved the zero-setback design, avoided a substantial redesign, and protected valuable interior space.
Good project records may seem unimportant until a date, decision, or earlier approval comes into question.
Understand the Requirements Created by the Design
Building closer to the property line gave us more usable space, but it also introduced stricter fire-protection requirements.
The enclosed stairwell was constructed with fire-resistant walls on all sides because of its proximity to the neighboring property. These requirements affected how the stairway and storage area could be designed and built.
Maximizing the footprint is not simply a matter of drawing a larger building. Each design decision may create corresponding code, construction, and budget implications that need to be understood together.
Investigate What Is Happening Below Ground
Drainage was already a persistent problem on the property, which had been developed over an old creek bed. Several gravity-fed drainage systems had been attempted, but the lot’s contours prevented them from working effectively.
Adding a new structure without solving that problem could have left water collecting beside it. Instead of repeating an approach the site would not support, we installed a sump pump at depth to collect pooling and residual water and mechanically discharge it from the low area.
Drainage, soil conditions, utility locations, and changes in elevation are less visible than a floor plan. They can be just as important to the project.
Define What the Property Can Support
The completed ground floor includes a garage large enough for a full-size vehicle, additional storage, a private office, a full bathroom, and indoor laundry. An enclosed stairway leads to a separate one-bedroom ADU with its own kitchen, living space, bedroom, and bathroom.
The homeowner gained rental income and the spaces he needed without remodeling his existing home.
The final design did not come from squeezing a standard ADU plan into the backyard. It came from understanding the homeowner’s priorities, verifying the property conditions, interpreting the applicable regulations, and resolving the technical constraints together.
That is the purpose of Project Development: determine what can be built, what it will require, and whether the project makes sense before construction begins.
