ADU Plans: Check the Lot, Resident & Permits Before Buying

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ADU plans are easy to browse and surprisingly easy to misjudge. A charming 400-square-foot cabin may make efficient use of its interior but require an awkward stair or ladder; a physically larger plan may have a simpler one-level circulation route. Either can fail on the actual property because of setbacks, trees, utilities or an incomplete permit package. The correct first step is to screen the site and the resident—not to choose the nicest exterior rendering.

Define what kind of ADU you are actually planning

An accessory dwelling unit is not simply any backyard building. Its permitted use, independent living facilities and relationship to the primary residence are defined by the relevant jurisdiction. Some projects involve converting an existing garage or part of a house; others are new detached dwellings. The City of Portland’s ADU permitting guide provides a concrete example of these different routes and their application requirements. Portland’s rules are an example for Portland property owners, not a US-wide size or setback standard.

Write down the use case before shortlisting plans: a long-term household member who needs independent cooking and bathing, a family member visiting occasionally, or another legally allowed dwelling arrangement. For a relative needing step-free access, the bed and bathroom route is at least as important as the advertised bedroom count. Do not assume a catalog label certifies wheelchair accessibility or legal rental use.

Two real small plans show why layout matters more than the headline size

Advanced House Plans includes the following designs in its ADU collection. The comparison uses its published individual plan specifications as checked October 27, 2026. Neither is represented here as locally pre-approved, engineered for your site, or independently reviewed for accessibility.

Sources: Kristine official plan page and Bluestone official plan page. The descriptions establish intended layout features, not an accessibility certification, approval to use a sleeping loft, or proof of buildable lot fit. A local professional must verify those questions.

Screen the property in the right order

1. Find the actual buildable envelope

Start with the site boundary, existing house, driveway, accessory structures, trees, slope, easements and property lines. Ask the planning or permitting office how setbacks, height, lot coverage and design rules apply to this address. A manufacturer’s 18-by-20-foot exterior dimension is not the same as a guaranteed 18-by-20-foot buildable slot. Depending on the project, fire separation, access routes, required clearances and utility corridors may narrow your options further.

2. Identify connections and site work

Map water supply, wastewater/sewer, drainage, electrical service and possible heating/cooling locations. The Portland guide specifically identifies utility connections, building and trade permits, and property-specific zoning checks as part of the ADU process. Those site costs can be substantial even when the stock drawing price appears low. Ask qualified providers for a site-specific scope rather than applying a made-up national allowance.

3. Ask what drawings the permit authority accepts

A floor-plan preview is not necessarily the permit set. A local reviewer may need site plans, foundation details, sections, structural information, energy compliance and mechanical or electrical documents. The City of Portland’s residential drawing checklist shows how plan reviewers use several drawing types to assess an actual project. The examples and scale guidance on that page are for Portland; the broader lesson is to ask your own office for its current submission checklist.

Stock plans versus locally pre-approved designs

Before paying for a catalog plan, search the actual jurisdiction for an official standard-plan or pre-approved-ADU program. Washington State’s Department of Commerce explains that permit-ready plans are designs already reviewed for consistency with participating local rules. That does not make them transferable to every city or lot.

Portland provides an especially practical caution: its official ADU page offers some free pre-approved detached designs, but explicitly warns they are not suitable for all lots. It also says all ADU construction projects need a site plan and notes that certain pre-approved bathrooms may still need changes for specified visitability requirements. This is why “pre-approved” is not identical to “permit issued” or “nothing left to design.”

Fit the person, not just the parcel

For day-to-day living, draw a line from entry to kitchen, bathroom, bedroom and laundry. Test door swings and space around furniture, and ask whether quiet areas are adjacent to windows facing the main house. If a parent expects to age there, give special attention to step-free access, bathroom turning space and staying on one level. These are design questions, not promises that a stock plan meets an accessibility standard. The AARP housing-accessibility policy resource identifies wide doors, maneuvering room and zero-step approaches as useful universal-design concepts.

Check whether the ADU will provide independent cooking and laundry facilities that the intended occupant needs. A small kitchenette or loft sleeping area may make sense for one situation but not another. Conversely, choosing a larger, expensive building just to gain square footage may be counterproductive if the layout is less practical.

Ownership, change requests and real costs

Separate drawing purchase from structural/site redesign and construction. Advanced House Plans describes optional plan changes on its collection pages, but availability and feasibility are plan-specific. Its single-build license restricts reuse and its digital-plan refund policy generally excludes returns. Confirm the permitted use, selected file format, precise alterations and current price in writing before paying.

If your goal is an attached suite within a shared household rather than a separate accessory dwelling, the legal and layout questions can change. Do not use an ADU purchase decision as a stand-in for choosing an attached room or shared-home suite; these are distinct layout and legal questions.

Six questions to answer before checkout

1. Does the local authority allow the intended dwelling use at this address, and under which current rules?
2. What exact width, depth and height fit after setbacks and other site constraints?
3. Where will water, wastewater, power and drainage connections go?
4. Can the intended resident safely use the sleeping and bathing layout every day?
5. Which required permit documents and local engineering are excluded from the stock set?
6. Is a qualifying, locally pre-approved alternative available—and what site review does it still need?

Method and geography: US-focused guide based on merchant plan listings, AARP, Washington State and Portland municipal materials inspected October 27, 2026. Portland is an explicitly local example, not a national permit rule. No site or ADU was physically inspected; confirm current rules with your own municipality.