None of these mistakes are exotic — they're the ordinary, avoidable ones that show up over and over in ADU projects. Catching them early is mostly a matter of asking the right questions before you're financially committed.
Skipping the feasibility check
Homeowners sometimes fall in love with a specific ADU type — usually a detached unit — before confirming their lot can actually support it once setbacks, existing structures, and any overlay zones (like a coastal or historic district) are factored in. A property review before you start designing anything saves you from discovering a dealbreaker after you've already spent money on plans.
Budgeting only for construction, not the whole project
A per-square-foot construction estimate doesn't include design and engineering fees, permit and impact fees, or site work like utility trenching and grading — together these commonly add meaningfully on top of the base construction number. See our ADU cost guide for the full breakdown of what's typically included and what isn't.
Picking the ADU type before understanding the trade-offs
A detached ADU, garage conversion, attached ADU, and Junior ADU each carry real differences in cost, timeline, and design flexibility. Choosing based on square footage alone, without weighing what you're giving up in speed or budget, is a common source of frustration mid-project. Our detached vs. attached comparison walks through the trade-off in more depth.
Not lining up financing before getting quotes
Getting contractor bids before you know how you'll actually pay for the project can waste everyone's time, especially if your financing option depends on projected rental income or existing home equity that takes time to document. Understanding your financing options early lets you scope the project to a budget you can actually fund.
Assuming every city's rules are the same
California sets a statewide floor for ADU rules, but individual cities can layer on their own procedures — pre-approved plan programs, specific fees, coastal or historic overlay requirements, or (in some cities) short-term rental restrictions. Assuming your neighbor's experience in a different city applies directly to yours is a common source of surprise. See our California ADU laws guide for the statewide baseline, and check whether your city has a dedicated city guide.
Planning rental income around assumptions instead of your specific market
It's tempting to plug in a rent estimate you saw online and build a return-on-investment story around it. Actual achievable rent depends heavily on your specific neighborhood, unit size, and condition — and in some cities, on rules that restrict how a unit can be rented (short-term vs. long-term). See our ADU rental income guide for how to think about this more carefully.
Only getting one contractor bid
A single bid gives you one data point, not a real sense of market pricing or approach. Comparing multiple proposals — from contractors who actually understand your ADU type and city — tends to surface differences in scope, timeline, and price that a single quote won't reveal on its own.
How to avoid most of these at once
Most of the mistakes above come from moving forward before you have a clear, honest picture of your property, budget, and options. A free property review is built specifically to surface these issues early — before you've spent money on plans, before you've committed to a contractor, and before you've made assumptions about rent or resale value that may not hold up.
Frequently asked questions
Skipping a feasibility check before committing to a specific ADU type or design. Confirming your lot, zoning, and any overlay requirements can support what you're planning — before spending on design work — avoids the most expensive version of this mistake.
Often, yes — especially earlier in the process. If you haven't signed a construction contract yet, there's usually still room to revisit your ADU type, financing plan, or budget assumptions. A licensed contractor or a fresh property review can help you assess where you stand.
Yes, that's largely the point of the free property review — we look at your property and goals early, flag anything that doesn't line up with what's realistic, and connect you with contractors in our network once there's a clear, sound plan to work from.