Structural and MEP design for 3 ADUs, San Francisco, CA  

S3DA Design provided structural and MEP engineering design for a modular ADU concept in San Francisco, California. The project includes three different ADU configurations, ranging from approximately 400 SF to 1,200 SF, with 1-bedroom, 2-bedroom, and 3-bedroom layouts built around a repeatable central living and kitchen core. 

S3DA’s Scope 

• Structural load analysis in accordance with the California Building Code and ASCE 7, including dead, live, roof, wind, and seismic loads  

• Gravity and lateral design for conventional wood-frame construction  

• Foundation design for slab-on-grade, raised crawlspace, and slope-condition alternatives  

• Full vertical and lateral load path verification  

• Structural plans including foundation plans, roof framing, shear wall layouts, sections, details, and notes  

• Mechanical design including heat load reports, floor plans, roof plans, refrigerant piping, condensate drainage, and schedules  

• Electrical design including lighting and power plans, panel schedules, calculations, and single-line diagrams  

• Plumbing design including domestic water, sanitary drainage, natural gas, roof rainwater design, risers, and schedules  

• Modular-compatible detailing to support efficient repeated deployment across multiple ADU types  

Key Challenges & Solutions 
A key challenge in this project was creating three ADU sizes from one modular design concept while maintaining structural consistency and efficient MEP coordination. S3DA approached this by engineering a flexible core layout that can expand outward with additional bedrooms and bathrooms, helping streamline design repetition without compromising code compliance or constructability. The team also accounted for multiple foundation scenarios, including slab-on-grade, raised crawlspace for flood-prone sites, and slope-adapted alternatives using piers or structural slabs. 

The Result 
The result is a flexible, permit-ready engineering solution for a repeatable ADU product line tailored to San Francisco conditions. By integrating Structural, Mechanical, Electrical, and Plumbing design under one team, S3DA helps reduce coordination issues, improve efficiency, and support faster deployment across multiple site conditions and finish variations.