What Actually Drives the Cost of an ADU

The Six Drivers That Set Your ADU Budget

Before touching any of the details, it helps to see the whole board. Nearly every ADU budget is shaped by six factors, and they interact with each other.

• Conversion vs detached
• Utilities and hookups
• Site conditions
• Size and finish level
• Design and drafting scope
• Permitting by jurisdiction

Conversion vs Detached: The Biggest Fork in the Road

The single largest decision is whether you convert existing space or build new. A garage or basement conversion reuses a foundation, a roof, and most of a shell that already exists. Those are some of the most expensive parts of any build, so keeping them typically pulls the budget down substantially. A detached new-construction ADU buys you more design freedom, better privacy, and often better rental appeal, but you pay for every system from the ground up: excavation, foundation, framing, roofing, and full utility runs.

The caveat is that conversions are only cheap when the existing structure cooperates. An old garage with a deteriorated slab, inadequate ceiling height, or framing that will not meet current code can absorb enough remediation work to erode the savings. A conversion should be evaluated, not assumed.

Utilities and Hookups: The Invisible Line Item

Utilities are where ADU budgets most often surprise people, because the costs are invisible until someone maps them. The questions that matter: How far is the ADU from existing water, sewer, gas, and electrical service? Does your electrical panel have capacity for a second dwelling, or does it need an upgrade? Will the jurisdiction allow you to share connections with the main house, or require separate meters and dedicated lines?

Every additional foot of trenching, every panel upgrade, and every separate meter adds cost, and the rules on what is allowed vary by jurisdiction and by utility provider. A detached ADU at the back of a deep lot can carry meaningful utility costs before a single wall goes up, while a conversion attached to the main house may need very little. This is a driver you want quantified early, because it is one of the few that can quietly rival structural costs.

Site Conditions: What Your Lot Adds or Subtracts

Two identical ADU designs can cost very different amounts on different lots. Slope is the classic example: a sloped site can require stepped foundations, retaining walls, and engineered drainage. Poor soils can trigger geotechnical reports and beefed-up foundations. Mature trees, existing hardscape, tight side yards that complicate equipment access, and drainage patterns all add or subtract work.

None of this is a reason to abandon a project, but it is a reason to walk the site with realistic eyes before falling in love with a floor plan. Good design responds to the lot instead of fighting it, and that response is often the difference between a smooth budget and an inflated one.

Size and Finish Level: Where Per-Square-Foot Logic Breaks

Bigger costs more, but not proportionally. Kitchens and bathrooms are the most expensive rooms in any dwelling, and every ADU needs them regardless of size. That fixed core means small units carry a higher cost per square foot, while added bedroom or living space is comparatively cheap area. Doubling the footprint does not double the price.

Finish level is the other half of this driver, and it is the one most within your control. Cabinetry, countertops, flooring, tile, fixtures, and appliances can swing a budget dramatically at the exact same footprint. A disciplined approach is to spend where finishes affect durability and rentability, and stay standard everywhere else.

Design, Drafting, and Permits: The Paper That Controls the Price

Design and Drafting Scope

Design fees depend on how much design you actually need. A pre-designed plan adapted to your lot sits at the low end of the range; our stock plans start at $799. Fully custom design costs more because it involves more hours, but it earns its keep on unusual lots, strict design review requirements, or when the ADU has a specific job to do, such as housing a parent with accessibility needs. Complexity adds scope too: structural engineering, energy compliance documentation, and site plans are sometimes required and sometimes not, depending on the jurisdiction and the design.

Our ADU Plans & Design service covers both paths, from adapting an efficient pre-designed layout to drafting a fully custom unit around your lot and your goals.

Permitting Complexity: The Jurisdiction Wildcard

Permitting is the driver you control least, which is exactly why it should be researched first. Plan check fees, impact fees, school fees, utility connection charges, required reports, and review timelines all vary by jurisdiction, and sometimes dramatically between neighboring cities. Some jurisdictions have streamlined ADU approval pathways and even pre-approved plan programs; others require multiple review cycles, and each cycle costs time and revision work.

Because these rules change and vary so widely, treat any general claim about permit costs with suspicion, including ours. The reliable move is to identify your specific jurisdiction's requirements before design begins, so the drawings are built to pass review the first time. For a deeper look at how rules shape the process, see our guide to ADU rules, costs, and timelines.

Where Design Fees Sit: Small Share, Big Leverage

Here is the part most budget conversations get backwards. Design and drafting are typically a small slice of total project cost, while construction, site work, and utilities consume the overwhelming majority. Yet almost every dollar in those large categories is determined by decisions made on paper, before anyone breaks ground.

The plan set decides whether the foundation is simple or engineered, whether utility runs are short or long, whether the layout wastes framing on hallways, and whether the permit reviewer approves the set or sends it back. Skimping on the smallest line item to save a little often costs multiples of that amount in the largest ones. Complete, coordinated, permit-ready drawings also let contractors bid the same scope, which is what makes bids comparable in the first place.

How to Budget for an ADU in Stages

You do not need to fund the whole project on day one. Staging the budget turns one intimidating number into a sequence of small, informed commitments, where each stage tells you whether the next one makes sense.

1. Feasibility first. Confirm what your jurisdiction allows on your lot: unit size, setbacks, height, parking, and whether conversion or detached is even an option. This stage costs mostly time and prevents the most expensive mistake of all, designing something you cannot build.
2. Scope the big drivers. Get a realistic read on utilities, site conditions, and structure quality if converting. These are the wildcards, so pin them down before finalizing a design direction.
3. Fund the design and permit set. This is the small-share, big-leverage spend. Complete drawings convert your project from a guess into a defined scope that a building department can approve and contractors can price.
4. Bid before you commit. Use the finished plan set to collect multiple contractor bids on identical scope. Only now do you have a real construction number, and only now should you lock financing and finish selections.
5. Hold a contingency. Whatever the bids say, reserve a cushion for the unknowns that surface once walls open and trenches are dug. Every experienced builder plans for this; homeowners should too.

Staged this way, the expensive commitment comes last, after the cheap information has been gathered. You can review our flat-fee drafting rates any time on our pricing page.

Frequently Asked Questions