Using a manufactured home as an accessory dwelling unit (ADU) in San Diego County is legal, affordable relative to site-built ADU construction, and increasingly popular. The rules have also changed significantly in the last two years. Here’s what applies in 2026 and what you need to know before you start.
What makes a manufactured home a valid ADU
California state ADU law (Government Code Section 65852.2) does not restrict ADUs to site-built construction. A manufactured home that meets all applicable building standards, is properly permitted, and is installed on a permanent foundation qualifies as an ADU.
The home must be:
- Built after June 15, 1976 with a valid HUD certification label
- Installed on a permanent foundation (required for real property conversion)
- Compliant with San Diego County’s design standards for manufactured homes, including conventional siding, roof pitch of at least 2:12, and eave overhangs of at least 1 foot
If those conditions are met, the manufactured home functions as an ADU for all permit and regulatory purposes.
Size limits and setback rules in 2026
State law in 2026 allows detached ADUs up to 1,200 sq ft. ADUs 850 sq ft or smaller (or 1,000 sq ft if they have more than one bedroom) cannot be denied based on floor-area-ratio or lot coverage standards. In practice, this means most residential lots in San Diego County can accommodate a manufactured home ADU without fighting zoning math.
Setbacks for detached ADUs in most cases are:
- 4-foot rear and side setbacks under state law
- High and very-high fire hazard severity zones require 4-foot setbacks with ignition-resistant construction materials. A large portion of unincorporated San Diego County falls in these zones.
Front setback requirements still apply. The ADU cannot reduce required parking below what the main home needs unless a parking exception applies.
Owner-occupancy is not required for most ADUs in 2026. California state law eliminated the owner-occupancy requirement for ADUs in 2020, and that rule remains in effect.
AB 1033: Selling your ADU separately
This is the rule change with the biggest financial implications.
AB 1033 allows ADUs to be sold separately as condominiums, independent of the main house. San Diego County (unincorporated areas) adopted it effective April 4, 2026. The City of San Diego adopted it in 2025.
What this means practically: if you build a manufactured home ADU on your property in an unincorporated area of San Diego County, you can potentially sell it as a standalone condominium unit rather than needing to sell the entire property. Some owner-occupancy and first-refusal rules are still being finalized as of mid-2026, so get current guidance before structuring a project around AB 1033 sale scenarios.
For buyers considering a manufactured home ADU as an investment or multi-generational housing tool, AB 1033 changes the exit options significantly. This is newer territory and worth discussing with a real estate attorney familiar with the county’s current implementation.
Impact fees: the big cost savings for small ADUs
Impact fees are fees charged by local agencies to offset the infrastructure burden of new development. For ADUs they can add up to significant money.
Under California law, ADUs 750 sq ft and under are exempt from most impact fees. This includes school impact fees, park fees, and transportation fees. For ADUs in this size range in San Diego County, the savings commonly run $10,000 to $30,000 compared to larger units.
An 800 sq ft manufactured home ADU is above the threshold and will incur some fees. A 700 sq ft or smaller ADU gets the full exemption. If you’re targeting the lowest total cost and your household needs can be met in a smaller footprint, staying under 750 sq ft has a real financial payoff.
What does a manufactured home ADU cost all-in?
The all-in cost for a manufactured home ADU in San Diego County typically lands in the $180,000 to $300,000 range, though complex sites can go higher. Here’s what drives the number:
The home itself: A new single-section manufactured home in the 600 to 900 sq ft range usually runs $80,000 to $150,000 from a dealer, including delivery and setup on an existing improved lot.
Foundation: $15,000 to $35,000 for a permanent foundation appropriate for the home size and site conditions.
Utility connections: connecting the ADU to the existing property’s water, sewer, and electrical systems typically runs $10,000 to $30,000. Some properties need panel upgrades or lateral extensions that cost more.
Permits: San Diego County PDS permits for an ADU project commonly run $5,000 to $15,000 depending on project complexity and which jurisdictional area your property is in.
Grading and site prep: flat lots with good access cost less. Sloped lots or tight access situations add cost.
Total project costs below $180,000 are possible on improved lots with utilities already close. Projects exceeding $300,000 typically involve complex sites, larger homes, or significant utility work.
Comparing manufactured home ADUs to site-built ADUs
Site-built ADU construction in San Diego County commonly runs $300,000 to $500,000 or more for a properly finished unit. A manufactured home ADU, built to HUD standards in a factory, achieves the same permitted outcome at a lower cost. The tradeoff is design flexibility. Manufactured homes come in standard floor plans; a site-built ADU can be designed to exactly match the main home’s architecture.
For buyers where cost and timeline matter more than custom design, the manufactured home path makes clear sense.
The permit process
The ADU permit process for a manufactured home mirrors the standard private-land installation process:
- Pre-application with San Diego County PDS to confirm the parcel qualifies and get guidance on what’s required.
- Permit applications for grading, foundation, and the ADU itself.
- Installation and inspection of foundation and utilities.
- HCD Form 433A recorded after installation to convert the home to real property.
- Final inspection and certificate of occupancy from the county.
Timeline from permit application to occupancy is typically six months to over a year depending on site complexity and permit backlog.
Can the ADU be rented out?
Yes. Owner-occupancy is not required for ADUs in California in 2026. You can rent the manufactured home ADU as a long-term rental. Short-term rental (vacation rental) rules vary by city and county ordinance, so check current short-term rental regulations for your jurisdiction before pursuing that path.
Start with a parcel check
Not every parcel in San Diego County is straightforward for a manufactured home ADU. Zoning, fire hazard designation, lot size, and existing improvements all affect what’s possible and how much it costs.
Land & Home SD helps buyers evaluate their property before committing to anything. Call (858) 925-5546 or visit our manufactured home ADU service page for a free initial conversation about your parcel.