Maximizing Urban Infill: 12-Unit ADU Addition

Maximizing urban infill: 12-unit ADU addition

June 9, 2025 | Written by: Tom Sears

ADU Features Design Challenges Design Solution Impact/Effect

Tom Sears is the Vice President of Architecture at ADU Geeks and a licensed architect with experience across multifamily, single-family, and hospitality projects. He has worked on developments ranging from 1,000 to 1,000,000 square feet, with construction budgets from $120 to $1,200 per square foot. With a strong foundation in both design and real estate, Tom brings a strategic, housing-first approach to ADU planning and execution across California.

ADU Features

This urban infill ADU project in San Diego transformed a standard 140’ x 50’ lot into a high-density residential site through strategic planning and code-aligned design. The site originally featured a 1,175 sq. ft. single-family home with 2 bedrooms, 2 bathrooms, and an attached garage.

Rather than demolish the existing home, the project retained it and added density through a garage conversion and a new multi-unit configuration. In total, the project delivered 13 residential units by leveraging the California ADU Density Bonus Program.

Key features include:

1,175 square feet existing residence with 2 beds / 2 baths
Garage conversion ADU added to existing structure
12 new ADUs constructed as part of a multifamily addition
Urban infill site measuring 140' x 50', with a 64' x 50' buildable area
Bonus density achieved through San Diego's ADU Bonus Program
Smart design preserved existing units and avoided affordable housing replacement requirements

This project exemplifies how to unlock maximum housing density on a small urban site—without displacing existing housing stock.

Design Challenges

This project was driven by a clear mandate: to achieve the highest and best use of the property, in its truest form. That meant addressing all four key development criteria:

1. Physically possible
2. Legally permissible
3. Financially feasible
4. Maximally productive

With only 64 feet of buildable depth and a site width of 50 feet, the available footprint was highly constrained. Achieving maximum density under these spatial conditions required a deep understanding of zoning, building code, and financial modeling.

Infill projects at this scale often face limitations related to required stairwells, unit efficiency, and municipal density regulations—especially when pushing the limits of what is financially viable.

Design Solution

To navigate these constraints and avoid triggering affordable unit replacement requirements, the client pursued a design strategy focused on efficiency and code-smart planning. The solution? A 12-unit addition designed to fit within a 64' x 50' buildable area, guided by an in-depth analysis of allowable density under the ADU bonus framework.

To optimize rentable space, the design team leveraged California Building Code Section 1006.3.4(1), which permits a single stair in certain multi-unit residential projects. This code interpretation allowed the team to eliminate one full stair core—increasing rentable square footage and unlocking more usable area per floor.

This key design move dramatically improved internal layout flexibility and supported higher unit efficiency without compromising safety or code compliance.

Impact/Effect

The results of this infill ADU project are nothing short of extraordinary in terms of spatial efficiency and density performance:

The use of exterior egress balconies—which are excluded from gross floor area—further contributed to the project’s space optimization strategy.
The final residential floor plate is 93.8% efficient, calculated using the formula: (Rentable Square Footage ÷ Gross Floor Area) × 100

This level of space efficiency is extremely rare in urban ADU and MDU development and may no longer be possible under future regulatory guidance.
The project not only met the four tests of development feasibility but also served as a model for code-aware, density-maximizing ADU design.

By thinking beyond traditional multifamily layouts and leveraging both state and local ADU incentives, this development proves what's possible when architecture, code knowledge, and policy expertise come together to unlock a property’s full potential.