What Plans Do You Need to Build an ADU in California?

If you are planning an accessory dwelling unit, one of the first questions is usually simple: what plans are needed to build ADU California projects that can actually get through plan check? The short answer is that most cities and counties want a complete permit drawing set that shows the property, the existing structures, and the proposed ADU clearly enough for reviewers to confirm code compliance.

In practice, that means more than a single sketch or floor plan. A typical ADU permit package includes existing-condition measurements, an ADU site plan, proposed floor plans, exterior elevations, building sections, and construction details. The exact list can vary by jurisdiction and by project type, but if you start with accurate as-built information, the rest of the design and permit process is usually smoother.

Why the existing-condition plans matter first

Before an architect, designer, or engineer can produce reliable ADU permit plans, they need to know what is already on site. That is especially important in California, where many ADU projects involve detached garages, additions to older homes, interior conversions, or backyard placement on lots with tight setbacks and limited access.

Existing-condition plans, often called as-built drawings or measured floor plans, document the home and structures as they actually exist today—not as someone assumes they were built. For ADUs, these drawings often become the base layer for design, zoning review, and coordination with consultants.

FastAsBuilt measures properties on site with laser tools, then senior drafters produce permit-ready CAD files in PDF and DWG format. If you are still getting familiar with the process, it helps to understand what as-built drawings are and why they are often the first step before design begins.

• They confirm current dimensions of the main house, garage, or accessory structure.
• They help identify wall locations, openings, stairs, and overall building footprint.
• They reduce errors when the proposed ADU is being laid out on the lot.
• They give your design team a clean CAD base instead of relying on rough hand measurements.

If your ADU will convert an existing space, such as a garage or part of the house, accurate existing drawings are even more important because the permit review will depend on what stays, what changes, and how the conversion is documented.

The core ADU permit plans most California projects need

Although each city can ask for slightly different sheet titles or submittal forms, most ADU drawings fall into a familiar set. Think of the permit package as a complete visual explanation of the project. Reviewers are not just looking for the new floor plan; they need enough information to understand how the ADU fits on the property and how it will be built.

• Cover sheet: project address, scope, sheet index, and basic project data.
• Existing as-built plans: measured plans of the current house, garage, or structure involved.
• ADU site plan: lot layout, property lines, existing buildings, proposed ADU location, access, and key dimensions.
• Proposed floor plans: room layout, doors, windows, fixtures, dimensions, and uses.
• Exterior elevations: each exterior face of the ADU, showing height and appearance.
• Building sections: cut-through views that explain floor-to-roof construction and heights.
• Construction details: specific wall, roof, foundation, or connection details.
• Structural, energy, mechanical, electrical, and plumbing sheets: when required based on project scope and your design team.

For many owners, the confusion starts when they hear “plans” and assume that means one page. In reality, ADU permit plans are a coordinated drawing set. If you are starting from an existing structure, you may also want to read about as-built plans for ADUs in California so you know what information should be captured before design work moves ahead.

The ADU site plan and why it is so important

If there is one sheet that almost always carries major weight in plan check, it is the ADU site plan. This drawing shows how the proposed unit sits on the lot in relation to property lines, the primary residence, driveways, walkways, fences, and other structures. It is where reviewers quickly assess placement and overall feasibility.

An ADU site plan often includes:

• Property boundaries and lot dimensions
• North arrow and scale
• Existing house footprint
• Existing garage or accessory structures, if any
• Location of the proposed ADU
• Setback dimensions from property lines and nearby structures
• Access paths, entries, patios, and exterior stairs when relevant
• Driveway and parking layout if your jurisdiction requests it
• Utility locations or notes if part of the design package

For detached new-construction ADUs, this sheet is often where lot constraints show up first. For garage conversion ADUs, it helps explain whether the footprint stays the same or expands. For attached ADUs, it clarifies how the new area connects to the existing house.

What the proposed floor plans need to show

The proposed floor plan is the sheet most homeowners imagine first, but permit reviewers expect it to do more than show where the bedroom and kitchen go. Good ADU drawings communicate function, dimensions, circulation, and the relationship between spaces.

A typical proposed ADU floor plan should show:

• Overall exterior dimensions and major interior dimensions
• Room names, including living area, kitchen, bedroom, bathroom, laundry, and storage
• Door and window locations
• Appliances, plumbing fixtures, and cabinetry layout
• Wall thicknesses and key construction notes
• Smoke and carbon monoxide device locations when part of the permit set
• Entry and egress paths

For a conversion project, plan reviewers usually need to compare existing and proposed conditions. That is one reason measured existing drawings are so useful. They help the design team create a clean before-and-after record instead of guessing from old real estate listings, faded plans, or tape measurements taken in a hurry.

If you are converting a garage, there are some extra considerations around documenting the existing structure accurately. This guide on getting as-built plans for a garage conversion ADU can help you understand that starting point.

Elevations, sections, and details: the sheets that explain buildability

Many homeowners focus on layout and forget that permit approval also depends on how the ADU looks and how it goes together. That is why elevations, sections, and details are standard parts of ADU permit plans.

Exterior elevations

Elevations show each side of the proposed ADU. They usually identify doors, windows, roof form, exterior finishes, and vertical dimensions. If your city is reviewing height, appearance, or consistency with the primary residence, these sheets become especially important.

Building sections

Sections cut through the building to show floor-to-ceiling relationships, roof slope, overall height, and major construction assemblies. They help reviewers understand the interior volume and confirm that the design matches the floor plans and elevations.

Construction details

Details zoom in on specific conditions that cannot be explained at larger scale. Depending on the project, that might include foundation edges, wall sections, roof connections, waterproofing transitions, stair details, or window and door conditions.

These sheets are where a rough concept becomes a real permit package. A city reviewer cannot approve construction based on a floor plan alone; they need enough information to understand the physical building and how the proposed work is intended to be executed.

How plan requirements change by ADU type

The plans needed to build ADU California projects are not always identical because the project type changes the drawing scope. A detached backyard unit, an attached addition, and a garage conversion all start from different conditions and create different permit questions.

• Detached ADU: usually needs a full new-construction drawing set, including comprehensive site, architectural, and structural sheets.
• Attached ADU: must show both the new ADU area and how it connects to the existing house.
• Garage conversion ADU: depends heavily on accurate existing-condition documentation, since the permit set must explain what parts of the garage remain and what changes.
• Interior conversion: often requires careful existing and proposed plans for the part of the home being reconfigured.
• JADU: may have a different scope from a full ADU, so the drawing set can differ depending on the local review process and the limits of the project.

If you are still deciding between project categories, it can help to compare ADU vs. JADU in California because the choice affects both design and permit documentation.

Custom work such as ADUs, SB9 projects, and tenant improvements is generally quoted individually because the needed drawing package depends on the actual structure, the lot, and the jurisdiction. If you already know you need measured plans to get started, you can review FastAsBuilt options or place an order through our order page.

What FastAsBuilt provides at the start of the ADU process

FastAsBuilt does not replace your architect or engineer, but we often provide the critical first layer they need: accurate field-measured as-built drawings and measured floor plans of the existing property. For many California ADU projects, that is the difference between designing from facts and designing from assumptions.

Our local crews serve Southern California, the Bay Area, and San Diego. We measure on site using laser tools, and our senior drafters create permit-ready CAD files delivered in PDF and DWG.

Our standard offerings include:

• 2D As-Built Plans: starting at $900 for up to 1,500 square feet, then $0.50 per square foot; includes 1 revision; typical delivery in 48–72 hours. Turnaround times are estimates and may vary based on project complexity and scheduling.
• 3D As-Built Plans: starting at $1,500, then $1.00 per square foot; includes a 3D model plus 2D floor plans, elevations, and sections; includes 2 revisions; typical delivery in 3–5 business days. Turnaround times are estimates and may vary based on project complexity and scheduling.

If you are budgeting the first step, this overview of as-built plan costs for an ADU can give you a clearer sense of the measured-drawing phase before full design fees enter the picture.

For owners, designers, and contractors, the practical benefit is simple: your ADU permit plans are easier to coordinate when the existing conditions are documented accurately from the beginning.

Common mistakes that delay ADU permit drawings

Even straightforward ADU projects can slow down if the initial information is incomplete. In California, delays often happen not because the project is impossible, but because the plans do not match the site or the set is missing basic coordination.

• Using old or incomplete plans: previous drawings may not reflect later remodels, additions, or garage alterations.
• Skipping measured existing conditions: rough dimensions can lead to design revisions and site conflicts.
• Treating the site plan as an afterthought: placement, access, and relationship to existing structures are central to review.
• Inconsistent dimensions between sheets: mismatches between floor plans, elevations, and sections can trigger corrections.
• Under-documenting conversion projects: when an existing garage or room is being converted, the permit set must explain both existing and proposed conditions clearly.

One of the simplest ways to avoid these issues is to start with dependable documentation. If you have just purchased a property or inherited unclear records, getting measured plans early can save time before design work ramps up.

Frequently asked questions

Do you always need as-built drawings before ADU design starts?

Not every project begins with a separate as-built package, but in many California ADU cases it is the smart place to start. If the ADU relates to an existing house, garage, or accessory structure, measured existing drawings help the design team work from accurate dimensions instead of assumptions.

Is an ADU site plan different from a floor plan?

Yes. A floor plan shows the interior layout of the building. An ADU site plan shows the building on the lot, including property lines, existing structures, and the proposed ADU location. Most permit packages need both because they answer different review questions.

Can old blueprints or county records replace new measurements?

Sometimes they are useful as reference material, but they should not automatically be treated as current conditions. Homes and garages are often altered over time, and permit records do not always match what is standing today. Measured field verification is the more reliable starting point.

What if your ADU is a garage conversion or interior conversion?

Those projects often need especially clear existing and proposed drawings. Since the structure already exists, reviewers need to understand what stays, what is removed, and how the converted space will function as a dwelling unit. Accurate as-built documentation is often one of the most valuable early steps.

Bottom line

The plans needed to build ADU California projects usually include more than one sheet: accurate existing-condition drawings, a detailed ADU site plan, proposed floor plans, elevations, sections, and the supporting permit documents required for the project type. If you begin with reliable field-measured as-built drawings, your architect or designer can build the permit set on a solid foundation—and that usually makes the path to approval more efficient.