What's Required to Build an SB 9 Duplex in California?

SB 9 gives many California property owners a path to add housing, but the real work starts when you turn the idea into a permit set. If you are planning an SB9 duplex, city reviewers, designers, and contractors typically need accurate information about the existing house, site, and utility conditions before they can prepare code-compliant plans.

This guide explains the practical side of SB9 duplex requirements, with a focus on the drawings and existing-conditions information that usually support a smooth submittal. It is written for owners in Los Angeles, Orange County, the Inland Empire, San Diego, and the Bay Area who need to understand what to gather first, what to ask for, and how measured as-built plans fit into the process.

What an SB 9 duplex project usually involves

An SB9 duplex project generally means creating two residential units on one lot where local rules allow that path under state law. In practice, that can take more than one form. You may be converting or expanding an existing single-family house into two units, keeping the current home and adding a second attached or detached unit, or replacing an older structure with a new duplex design.

The exact approval path depends on the property and the city, but the common thread is documentation. Reviewers need to understand what is already on the lot, how the proposed units relate to setbacks and access, and whether the design fits the site constraints. That is why the first step is often not architectural design, but measuring what exists now.

If you are still comparing state rules with local procedures, it helps to start with an overview of what SB 9 requires in California. Once you know your property is a candidate, the next question is usually whether your current plans are accurate enough to build from.

Why accurate existing-condition plans matter for SB9 duplex requirements

Many owners assume they can use an old listing sketch, appraisal layout, or a decades-old plan set. For SB9 plans, that usually creates delays. Existing structures often differ from past records because of remodels, enclosed patios, moved doors and windows, garage conversions, or field conditions that were never updated on paper.

Accurate existing-condition plans help your design team answer the basic permit questions early:

• What is the current footprint of the house and any accessory structures?
• Where are the existing walls, openings, stairs, and major built-in features?
• How much usable area is already on the lot?
• What can be reused, and what must be demolished or reconfigured?
• How should proposed unit separation, access, and circulation be laid out?

For many projects, measured plans become the base layer for the architect, designer, engineer, and permit consultant. If the base file is wrong, every later drawing can be wrong too. That is one reason owners often order as-built drawings before design begins, especially when no reliable CAD files exist.

The baseline documents you may need before SB9 plans are drafted

Every city package is a little different, but most SB9 duplex requirements start with a core group of existing-condition documents. These are not all produced by the same person, and knowing the difference helps you avoid ordering the wrong service.

Measured as-built floor plans

These show the current layout of the existing building as it stands in the field. FastAsBuilt measures on site with laser tools, then senior drafters prepare permit-ready CAD files in PDF and DWG format. For an existing house that will be converted, expanded, or partially retained, this is often the most important starting document.

Exterior elevations and building sections

If the project involves major rework to the existing structure, height comparisons, or additions tied into the current home, elevations and sections can save time. They help the design team understand rooflines, floor relationships, and wall conditions before new work is drawn.

Site plan inputs

A site plan usually needs the lot layout, building footprint, distances to property lines, access points, and other visible conditions. In some cases, your designer may build this from multiple sources, including measured building data and survey information where needed.

Survey or lot information

Not every owner knows when a survey is needed. That question comes up often with lot lines, setbacks, easements, and split-lot scenarios. If your project also involves dividing the parcel, review whether you need a survey for an SB 9 lot split and confirm the city’s expectations early.

Photos of existing conditions

Clear field photos support designers and consultants who may not attend the first site visit. Photos help document utility locations, slopes, roof form, exterior access, windows, doors, and any unusual conditions that should appear in the permit set.

When these materials are assembled at the start, your SB9 plans usually move faster from concept into coordinated drafting.

What measured as-built drawings typically include for an SB9 duplex

Measured as-built drawings are not the final permit design, but they are often the foundation for it. For an SB9 duplex, the right as-built package depends on whether you are keeping an existing house, adding onto it, or rebuilding more extensively.

A typical 2D as-built package can include the current floor plan with room layout, walls, door and window openings, and major dimensions captured from an on-site field measure. FastAsBuilt’s 2D As-Built Plans start at $900 for up to 1,500 square feet, then $0.50 per square foot, include one revision, and are typically delivered in 48 to 72 hours.

Turnaround times are estimates and may vary based on project complexity and scheduling.

For more complex projects, a 3D package can be useful because it includes a 3D model plus 2D floor plans, elevations, and sections. FastAsBuilt’s 3D As-Built Plans start at $1,500, then $1.00 per square foot, include two revisions, and are typically delivered in 3 to 5 business days.

Owners planning unit reconfiguration, second-story work, or a full redesign often want to understand what is included in as-built drawings before ordering. The main point is simple: the more accurately the existing conditions are documented, the easier it is to create dependable SB9 plans.

How SB9 duplex requirements affect the kind of plans you should order

Not every property needs the same starting package. The right level of documentation depends on the scope of work.

• Existing house converted into two units: Start with measured floor plans at minimum. If the roof, exterior walls, or split-level conditions are complicated, add elevations and sections.
• Existing house plus a new second unit: You usually need accurate existing plans for the current home so the designer can coordinate setbacks, access, open area, and how the second unit fits the lot.
• Demolish and rebuild as a duplex: Existing plans may still be helpful for permit background, demolition planning, and utility coordination, but site and lot information becomes especially important.
• SB9 plus other housing strategies: If you are also comparing an ADU path, JADU option, or phased buildout, the existing-condition package should support those comparisons without requiring repeated field visits.

Some owners begin with only the floor plan, then discover later that their architect also needs exterior elevations or sections. If your project has visible grade changes, older additions, attached garages, or irregular massing, it is often more efficient to capture the broader set of conditions from the start.

Common California issues that can slow an SB9 duplex submittal

SB 9 is statewide, but permit review is still local. In California cities and counties, the delays often come from missing or inconsistent existing-condition information rather than from the basic housing concept alone.

Common examples include:

• Plans that do not match the actual footprint of the building
• Unclear distances between structures and property lines
• Old garage or patio conversions not reflected in current drawings
• Missing elevations for projects that change the exterior significantly
• No clear base drawing in DWG for the design team to build on
• Conflicts between owner sketches, county records, and field conditions

These problems are especially common when a recently purchased home has limited documentation. If that sounds familiar, you may want to review as-built drawings for a house you just bought to see how owners rebuild the missing record before planning improvements.

Another issue is timing. Owners sometimes wait to gather existing plans until after they hire a designer, but the designer cannot price or scope the work accurately without a dependable base. That can lead to extra site visits, redesign, or permit comments that could have been avoided.

How FastAsBuilt supports the early planning stage

FastAsBuilt focuses on field-measured as-built drawings and measured floor plans across California. Local crews serve Southern California, including Los Angeles, Orange County, and the Inland Empire, as well as San Diego and the Bay Area.

The workflow is built for permit preparation. We measure on site with laser tools, then senior drafters produce permit-ready CAD files delivered as PDF and DWG. That means your architect or designer can begin with a clean digital base instead of tracing a rough sketch or relying on outdated records.

For straightforward houses, 2D plans are often enough to launch an SB9 duplex concept. For more involved reconfiguration, vertical additions, or major exterior redesign, a 3D as-built package can give the design team more context up front. Commercial properties, ADUs, tenant improvements, and other custom scopes are quoted individually, and the same custom approach applies when an SB9 project has unusual conditions.

If you are ready to move, you can order measured as-built plans online and get the field documentation process started before design decisions stall out.

How to prepare before ordering SB9 as-built plans

A little preparation can make your site visit faster and your deliverables more useful. Before you order, gather whatever records you already have, even if they are incomplete. Old permit drawings, assessor sketches, listing floor plans, and contractor markups can all help your team understand the property history, even though they should not replace field measurement.

You should also think about the end use of the plans. Ask yourself:

• Will the existing house stay largely intact, or be heavily reconfigured?
• Does the designer need only a floor plan, or also elevations and sections?
• Are there garages, porches, additions, or detached structures that should be included?
• Is the project only an SB9 duplex, or are you also exploring an ADU or lot split path?
• Do you need CAD files for a permit set consultant, architect, or structural engineer?

It also helps to ensure the property is accessible on measurement day. Clear access to all rooms, garages, side yards, and exterior perimeter areas reduces follow-up questions. If portions of the site are locked, occupied, or hard to reach, mention that in advance so the scope can be planned correctly.

For owners comparing costs before they begin, a practical next read is as-built drawings cost in California, which explains how pricing is commonly structured for measured documentation.

Frequently asked questions

Do SB9 duplex requirements mean you need as-built drawings first?

Not in every case, but accurate as-built drawings are often the smartest first step when an existing house will remain, be converted, or be expanded. They give your design team a reliable base file and reduce the risk of permit comments caused by outdated or inaccurate plans.

Are old floor plans from a listing or appraisal enough for SB9 plans?

Usually no. Those documents can be useful background, but they are rarely detailed or current enough for permit design. Field-measured plans are far better when room sizes, wall locations, door and window positions, or previous remodels affect the proposed duplex layout.

Should you order 2D or 3D as-built plans for an SB9 duplex?

It depends on scope. A 2D package is often sufficient for a simpler project focused on layout and basic design planning. A 3D package is more useful when the project involves elevations, sections, roof form, split levels, or a more complex redesign of the existing structure.

Can FastAsBuilt provide plans for SB9 projects anywhere in California?

FastAsBuilt provides field-measured as-built drawings and measured floor plans across California, with local crews serving Los Angeles, Orange County, the Inland Empire, San Diego, and the Bay Area. For custom scopes related to duplexes, ADUs, tenant improvements, or commercial work, the project can be quoted based on the property and deliverables needed.

Bottom line

SB9 duplex requirements are easier to navigate when you begin with accurate existing-condition information. Before your designer can produce strong SB9 plans, you usually need a dependable record of the current house and site. Measured as-built drawings help create that record, reduce guesswork, and give your team a practical base for permit-ready design in California.