Design-build ADU vs hiring an architect: which route fits

Two ways to build an ADU

Every ADU goes through the same work: design, engineering, the permit, then construction. What changes is who you hire for each part and how many contracts you sign.

The legal rules don't change with the model: who may design the building and what the construction contract must say are the same under both.

How a design-build ADU works

A design-build firm usually starts with a site visit and a feasibility check, then designs the unit, prepares or coordinates the plan set, submits it for permits and builds it. Many offer a menu of standard floor plans they've built before, plus custom design at extra cost.

Treat the feasibility step as a test of the firm. A useful one tells you, in writing, where the unit can go on your lot, how big it can be, how water, sewer and power will reach it, and which costs are still unknown. A vague feasibility report followed by a request to sign a full construction contract is a reason to slow down. Ask for a sample plan set and a sample construction contract from a past project before you commit to the design phase.

Who draws the plans in an ADU design-build firm

California lets anyone, including a contractor, prepare plans for a wood-frame building of up to two stories and a basement, and for buildings of up to four units. Most ADUs qualify, so a builder's in-house designer can legally draw them. Beyond that exemption, the California Architects Board says a contractor may design non-exempt work only under an architect's or engineer's supervision, with that professional signing the plans.

If a design-build company offers “architectural” services or uses “architect” in its name, the board requires a licensed architect in management control who is an owner, officer or employee, and the architectural work must be done under an architect's responsible control. If a firm says it has an architect on staff, ask for the name and look up the license.

Pros of design-build

• One point of responsibility. If the plans and the field don't match, the builder can't blame the designer.
• Cost feedback during design. The people pricing the job are in the room while you choose sizes and finishes.
• Less coordination for you. The firm manages engineering, the energy report and plan-check corrections.

Cons of design-build

• No bid comparison on the same plans. By the time you see a construction price, you're usually invested in that firm.
• Plans may not travel. If the relationship ends after design, ask whether you can take the plans to another builder, and on what terms.
• One company holds all the risk. If it fails, design and construction stop together. See how to hire an ADU contractor for ADU companies that closed mid-project.

How the architect route works

In design-bid-build, you hire a licensed architect or a building designer, and they produce a permit-ready plan set with structural and Title 24 energy work. You (or they) submit for the permit. Then you send the same plans to several builders for bids and sign with one. See ADU architects and designers for who can legally draw your unit and what it costs.

Comparing bids on the same plans

Identical plans make bids comparable, but only if every builder prices the same scope. Line the bids up by item and look for what each one leaves out. The state's contractor board singles out trenching for water, sewer and power as easy to leave out of an ADU bid. Other items to check: permit and impact fees, demolition, grading, the solar system, appliances and the final cleanup. Ask each builder to list allowances (budget amounts for finishes you haven't chosen yet), because a low allowance makes a bid look cheaper than it is.

A bid far below the rest deserves a question, not a celebration. Ask the builder to walk you through it line by line.

Pros of hiring an architect first

• Real price competition. Builders bid on identical drawings, so you compare like with like.
• An advocate on your side. The designer works for you, not for the builder, and can answer the builder's questions during construction if you pay for that.
• More design freedom. You aren't limited to one builder's standard plans or methods.

Cons of hiring an architect first

• Budget surprises come late. A design priced only after it's finished can come back over budget and need redrawing.
• Two relationships to manage. Disputes between the plans and the builder land on you.
• A separate design bill. Block Renovation's June 2026 Sacramento guide puts design, engineering and Title 24 work for a 600-square-foot unit at $14,000 to $20,000, and Snap ADU (April 2026) estimates custom plans from an architecture firm at $15,000 or more. Both are builders' estimates, not official figures. Design-build firms also charge for design, sometimes within the overall price.

Design build vs. architect: side by side

A middle route: hire your own designer, but bring one or two builders in to price the design before it's finished. You keep the plans and the bid comparison while catching budget problems early. Another: start from a pre-approved ADU plan, which cuts the design phase for either model.

Contracts and payments

The architect's contract

A licensed architect must use a written contract, signed before work starts unless you knowingly agree in writing otherwise. The board lists the required contents: the services, the amount and method of payment, both parties' details, how added services are handled and how either side can end the contract.

The construction contract

Whichever model you pick, the building contract is a home improvement contract. The Contractors State License Board says ADU construction is a home improvement project that requires a written Home Improvement Contract. State law sets what it must include: the contractor's license number, the price, a description of the work and materials, start and finish dates, a schedule of progress payments in dollars, a Mechanics Lien Warning and your cancellation notice.

• Deposit. No more than $1,000 or 10% of the contract price, whichever is less, and no exception for special-order materials.
• Progress payments. Each payment may not exceed the value of work done or materials delivered.
• Cancellation. You can cancel within 3 business days of signing, or 5 days if you're 65 or older.
• Subcontractors. Since January 1, 2026, a home improvement contractor must give you subcontractors' names, contact details and license numbers on request.
• Liens. Collect lien releases with each payment.

With design-build, ask whether design is billed under a separate design agreement or inside the construction contract, and what each payment buys. For the board's sample milestone schedule, see how to hire an ADU contractor.

Which fits your project

• Standard detached ADU or garage conversion, tight timeline: design-build, or a pre-approved plan with competing bids.
• Attached ADU, two stories or a hillside lot: an architect or experienced designer first, with a builder pricing the design early. See two-story ADUs and attached vs. detached ADUs.
• Tight budget, want the lowest price: finished plans and three or more bids, so builders compete on the same scope.
• Little time to manage the project: design-build, with payments strictly tied to finished work.

For the steps that follow either way, see the ADU construction process and how long it takes to build an ADU.

Red flags in either model

• Large payments before work starts. Whether the invoice says “design,” “plans,” “permits” or “materials,” the construction deposit is capped by law.
• A request that you pull the permit as owner-builder. If you sign as owner-builder, the City of San Diego's form makes clear you take on the contractor's responsibilities, including liability for uninsured workers.
• “Architect” in the marketing, no licensed architect behind it. Ask for the name and license number, then check it.
• The board's classic warning signs. The contractor board lists cash-only deals, pressure to sign right away, a bid far below the others and a contractor who says you don't need a written contract.

Questions to ask

For a design-build ADU company

• “Who draws the plans, and are they licensed?” If an architect or engineer is named, look up the license.
• “What does the design phase cost, and is it a separate agreement?”
• “If I don't build with you, can I keep the plans?”
• “When will I get a fixed construction price, and what's excluded?” Site work and utility connections are common exclusions.
• “Is the deposit $1,000 or less, and is every payment tied to finished work?”

For an architect or designer

• “Will you design to my construction budget, and how will you check it?”
• “Do your fees include engineering, the Title 24 report and plan-check corrections?”
• “Will you help me compare bids and answer builders' questions during construction?”

Before you sign with any builder, check a contractor's license and compare verified ADU builders in San Diego. For the full budget, see what an ADU costs in San Diego.

Common questions