Streamline Design & Permitting, a San Diego County pre-construction services firm specializing in accessory dwelling unit design and permitting, today released an advisory identifying five pre-construction mistakes commonly seen in ADU permit applications across the region.
More than 2,100 ADU permit applications were processed in San Diego in 2024, and additional regulatory changes took effect in 2026. The advisory outlines the following five issues observed in local ADU projects.
Mistake #1: Skipping the Feasibility Assessment
Zoning overlays, setback requirements, slope, easements, and utility capacity determine what can be built on a given lot. While California allows at least one ADU on most single-family lots, site conditions determine what is actually feasible. A feasibility review conducted before design work begins identifies these constraints early in the process.
Mistake #2: Relying on Outdated Regulatory Information
California has amended its ADU laws every year since 2017, with additional changes in 2026 affecting permit timelines, Junior ADU (JADU) owner-occupancy rules, and coastal zone protections. Applications based on regulatory information that is even a year old may contain incorrect setback calculations, outdated fee assumptions, or designs that no longer meet current requirements. The California Department of Housing and Community Development (HCD) maintains the state’s official ADU program page, which tracks legislative updates and is the authoritative reference for current state ADU law. Homeowners and design professionals are advised to confirm requirements against HCD’s published guidance before finalizing project plans, rather than relying on prior years’ regulations.
Mistake #3: Submitting an Incomplete Permit Application
California’s 2026 ADU rules established review timelines that apply to both permitting agencies and applicants. An incomplete submission gives reviewers grounds to request additional information, which extends the approval process. Application packages that coordinate architectural plans, structural engineering pages, and Title 24 energy compliance documentation are less likely to receive first-cycle rejections.
Mistake #4: Overlooking Ownership and Investment Structure
In family ADU projects, for example, a unit built on a parent’s property, or a backyard cottage funded on a child’s lot — the question of how to document a contributing party’s investment is sometimes left unresolved until after construction begins. Options include a title adjustment, a recorded instrument, or a condominium conversion pathway. Homeowners in these situations are advised to consult appropriate legal or financial advisors before finalizing a project’s design.
Mistake #5: Choosing an ADU Type That Does Not Match Long-Term Goals
A detached ADU, a junior ADU, a garage conversion, and an attached addition carry different cost profiles, approval pathways, and long-term flexibility. Homeowners who select an ADU type based solely on upfront cost sometimes find that the resulting unit does not generate the intended rental income or requires additional upgrades to support aging-in-place use.
About Streamline Design & Permitting
Streamline Design & Permitting is a San Diego County pre-construction services firm specializing in accessory dwelling units, whole-house remodels, residential additions, code compliance documentation, and commercial tenant improvements. The firm serves homeowners, investors, and developers throughout San Diego County, Los Angeles, southern Orange County, and southwestern Riverside County.