Please Advise: ADU Cost Components Breakdown?

[Emil Pinlac]

Would anyone be able to give a quick breakdown of the various costs associated with planning and building an ADU in either your backyard, or as a garage-conversion in California? I'm talking about modest-sized JADUs or ADUs in the 400-600 sq. ft. range.

I've been considering building or converting a garage into these units, for a long-term, buy & hold real estate strategy. But I just wasn't sure where to start, or how to properly save/consider the various costs associated with them. 

[Whitney Hill]

As a good rule of thumb, figure on $275-325/sqft for all-in ADU project costs…so an 500 sqft 1BR standalone ADU would typically cost around $160K. You can think of costs in two phases… Pre-construction and Construction:

• Pre-construction costs: $10-15K
• -Design: if using a stock plan, the design work costs about $5K. If going with a full custom plan (e.g. you have a unique lot layout that will require a customized layout)… closer to $10K, including construction drawings, title 24 energy report & electrical layout, permit processing
  
• -Permits: $5K is typically what the total costs to the city will run for an ADU, even with the waived fees
• Construction costs: $250/sqft + $10K
• -Building costs: typically $250/sf to start, for vertical building only. This is higher than for single family homes, since the cost is spread out over a much smaller footprint (e.g. still a kitchen+bath, but only 1-2 BRs).
• -Site work: typically $10K is a good number to start with, which includes preparing the building pad, connecting utilities, plus a small amount of landscape and concrete work.
• Other costs (not typically required): Additional structural engineering, Civil engineering, Soils engineering

Also, the cost would be around $75K for a standard garage conversion project of about 400 sqft....$8-10K for plans/design/engineering, $3-5K permits/fees, rest construction costs.