Central Coast Project Planning

ADU Planning and Permitting in San Luis Obispo County

Central Coast Project Planning (CCPP) helps homeowners plan and permit Accessory Dwelling Units (ADUs) throughout San Luis Obispo County. An ADU — sometimes called a granny flat, in-law suite, guest house, or casita — is a secondary residential unit on a property that already contains a primary home. Recent changes to California state law have opened up ADU development to nearly all residential properties, and as of June 2025, San Luis Obispo County may allow up to three ADUs per parcel. This shift is largely driven by the state's ongoing housing affordability challenges.

There are several practical reasons homeowners consider adding an accessory dwelling unit. Because the land is already owned, there is no additional land acquisition cost. ADUs also tend to add more value to a property than they cost to build, and their smaller footprint generally makes them more affordable to construct than a full-sized home.

ADU Types: Custom, Stock Plan, and Manufactured

When planning an ADU in San Luis Obispo, there are three main construction approaches to consider:

1. Custom ADU: A stick-built, fully custom accessory dwelling unit designed from the ground up. This option allows for the most flexibility in design and is well-suited for properties with unique site constraints. It requires a complete set of architectural, structural, and mechanical/plumbing/electrical plans. Custom ADUs are the most expensive option and typically take the longest to permit — generally 9 to 12 months.
2. Stock Plan ADU: Both the City of San Luis Obispo and San Luis Obispo County offer pre-reviewed stock plan designs for stick-built homes. These can reduce design costs and speed up the permitting timeline. However, site-specific work is still required, including grading, drainage, utility design, and various reports and studies. Construction costs are not significantly lower than a custom build.
3. Manufactured ADU: Manufactured homes are built in a factory, inspected before delivery, and then installed on-site. This approach tends to be the most affordable and has the shortest overall project timeline. The trade-off is fewer configuration options and more limited lending choices.

A general comparison of the three approaches:

1. Custom — Unlimited design potential, best for difficult or unique properties, but the most expensive with the longest permitting timeline.
2. Stock Plan — Reduced design cost and faster permitting, but limited floor plan options and minimal customization.
3. Manufactured — Most affordable and easiest to manage, but limited configurations and lending options.

The Permitting Process and Why Specialist Help Matters

Permitting an ADU in San Luis Obispo involves multiple steps and a range of technical requirements. The general process includes:

1. Evaluating property feasibility
2. Developing a conceptual site plan
3. Coordinating with third-party consultants for required reports and studies
4. Completing full construction documents
5. Submitting to the local agency and responding to comments
6. Receiving project approval

The permitting authority requires a complete construction document set, including scaled CAD drawings and a range of technical studies. Even with manufactured or stock plan ADUs — where home plans are provided — additional consultant work is typically needed. This can include surveys, soils reports, percolation tests, grading and drainage plans, septic design, foundation design, utility layout, fire access review, and WUI (Wildland Urban Interface) compliance, among others.

For most homeowners, attempting to navigate this process independently is not practical. A permit specialist like CCPP manages the planning, drafts construction documents, coordinates with consultants, and guides the project through to approval. For a manufactured or stock plan ADU, permitting typically takes 4 to 6 months. A custom ADU generally requires 9 to 12 months. Budgeting for design, consultants, and agency fees typically falls between $40,000–$50,000 for manufactured or stock plan ADUs, and $60,000–$70,000 for custom projects.

Before starting, homeowners are advised to establish a realistic budget, consult an ADU specialist early to identify any site or zoning constraints, and — if financing is needed — speak with a lender familiar with ADU projects. Site conditions such as power access, water supply, septic needs, fire zone classification, and foundation requirements can all affect cost and timeline, and are worth assessing early in the process.

Central Coast Project Planning works with homeowners across San Luis Obispo County to move ADU projects from initial concept through to permitted approval.