An ADU plan is the blueprint that turns your backyard into livable space. Here’s what a complete plan set includes, what it costs, how to choose the right one for your lot, and why starting with a ready-to-permit ADU plan can save you months and tens of thousands of dollars.
IN THIS GUIDE
Accessory dwelling units (ADUs), sometimes referred to as in-law units, granny flats, guest houses, or backyard cottages, have become an increasingly popular solution for property owners in California and across the country. With the rising demand for affordable housing and new construction opportunities, having access to well-designed ADU plans and pre-approved plan sets is now more important than ever.
An ADU is a self-contained housing unit located on the same lot as a main home. It can be attached, detached, or integrated within an existing structure. ADU development not only offers an affordable way to increase housing supply but also provides property owners with significant savings and opportunities for rental income and community development.
Choosing from a selection of pre-designed or pre-approved ADU plans helps streamline the process, making it easier to move from initial concept to construction phase. Pre-approved plans from your city or county can remove many obstacles and delays often associated with custom architectural plans. An ADU expert may have a bigger selection of pre-designed plans, that may work better for your lot than pre-approved ADU plans that tend to be rectangular.
An ADU plan is a complete set of construction documents that your city’s building department needs before it will issue a permit for an accessory dwelling unit. Think of it as the instruction manual for your entire project — without it, no contractor can break ground and no inspector can approve the work.
A complete ADU plan set — sometimes called an accessory dwelling unit plan package — typically includes architectural floor plans with room dimensions and window placements, exterior elevations showing all four sides of the building, a structural engineering package with foundation and framing details, Title 24 energy compliance calculations (required in California), and a site plan showing exactly where the ADU sits on your lot relative to setbacks, utilities, and the primary residence.
Some plan sets also include electrical and plumbing layouts, HVAC specifications, and a landscape or grading plan if your lot has a slope. The more complete the plan set, the smoother your permit review will go — and the fewer change orders you’ll face during construction.
Every ADU project begins with the first step: matching your available space and site conditions with an appropriate house plan. Most pre-approved ADU plans include:
These features empower property owners to easily select a design, customize it as allowed, and proceed by following plans through the local planning division and safety division for permit approval.
Note: These ADU plans are useful even when you are opting for a custom design for your ADU. They clearly show your designer what the city expects to see in the construction plan set.
ADU floor plans come in four main size brackets, each designed for a different use case. The right size depends on who will live there, how you plan to use the space, and how much buildable area your lot allows.
These compact units work well as rental studios, home offices with a bathroom, or guesthouses. A typical layout fits a bedroom (or open sleeping area), a full bathroom, and a kitchenette. Footprint: roughly 20 ft × 25 ft. Popular with homeowners who want rental income without giving up much yard space.
The sweet spot for aging-in-place units and long-term rentals. Enough room for a proper one-bedroom with a full kitchen and living area, or a tight two-bedroom layout. This is where most first-time ADU builders land — big enough for comfortable full-time living, small enough to keep construction costs manageable.
A 750–1,000 sq ft ADU house plan gives you a true two-bedroom, two-bathroom home with an open-concept living area and a full-size kitchen. Three bedrooms are possible at the upper end of this range, though rooms will be compact. This size commands the highest rental rates and works for small families.
California’s statewide ADU law permits accessory dwelling units up to 1,200 sq ft. At this size, you get generously proportioned bedrooms, a full laundry area, and enough kitchen space for serious cooking. These units often appraise and rent as well as many primary residences.
HomeWiP offers ready-to-permit plans in each of these size brackets.
Minimal size of an accessory dwelling unit in California by law is 150 sf ft. We are able to fit a bathroom and small kitchenette into a 200 sf ft studio. The most expensive price per square foot.
This one bedroom one bathroom can be built with a flat roof or with a more traditional pitch roof depending on the wind and snow load in your location.
This two bedroom one bathroom ADU has a primary en suite bathroom and an additional bathroom with access from the living area.
This two bedroom two bathroom ADU has a primary en suite bathroom and an additional bathroom with access from the living area. The kitchen has an option to become a separate room for those who like to cook spicy food.
There are two separate costs to think about: the plan itself and the construction it enables. Most homeowners are surprised by how much the design phase costs when they start from scratch.
Hiring an architect to create a custom ADU building plan from scratch typically runs $15,000–$30,000 and takes 3–6 months. That covers schematic design, design development, construction documents, and structural engineering — all before you submit a single permit application.
Ready-to-permit plans eliminate most of this cost and timeline. Because the architectural drawings, structural engineering, and energy calculations are already done, you skip straight to the permit application. Most homeowners save $15,000–$30,000 in design fees and 3–5 months of waiting.
Site-Built ADU (stick-frame on your lot): $200–$350 per sq ft in most California markets. A 750 sq ft unit runs roughly $200,000–$260,000 all-in, including framing, plumbing, electrical, HVAC, finishes, and permit fees. Foundation is site-specific in most cases, and will need to be estimated on demand. Urban areas with complex utility connections push toward the higher end.
Prefab / Modular ADU: $200–$250 per sq ft. The unit is factory-built, trucked to your site, and craned into place. Faster timeline (4–6 months vs. 8–12 for site-built), but you’ll need a crane-accessible path and a prepared foundation. A 600 sq ft prefab ADU typically costs $170,000–$190,000. This cost does not include foundation, utility connection, site work, transportation and crane costs.
Garage Conversion: $200–$270 per sq ft. Converting an existing garage into a living unit is the most affordable path because the foundation and walls already exist. A typical two-car garage conversion (400–500 sq ft) costs $100,000–$120,000. Additional work on foundation and utilities is often required.
California cities and counties like Alameda and Santa Clara are leading the way, offering “shelf ready” and permit-ready ADU plans through community resource centers and accelerators. These programs have created a gallery of pre-designed plans and floor plans that meet local regulations and building standards.
Opting for pre-approved ADU plan sets provides several advantages:
Permit Approval Efficiency: Using a selection of pre-approved plans shortens the streamlined permit process, saving time and pre-construction costs.
Significant Savings: Reduced need for custom site-specific design work and structural calculations often means lower architect and engineering fees.
Impact Fees and Regulations: Many counties and cities have adjusted impact fees for ADUs, making new construction and ADU projects even more accessible.
A ready-to-permit ADU plan set is a complete package of construction documents that your city’s building department can review immediately. It includes everything needed for permit submittal: architectural floor plans and elevations, structural engineering calculations, and Title 24 energy compliance reports.
When you start from scratch with an architect, the typical timeline looks like this: 4–8 weeks for schematic design, 4–6 weeks for design development, 4–8 weeks for construction documents, then 2–4 weeks for structural engineering. That’s 3–6 months before you even apply for a permit.
With a ready-to-permit ADU building plan, you skip straight to the permit application. Your architect or designer adapts the plan to your specific lot — adjusting setbacks, utility connections, and site orientation — but the core design and engineering are already complete.
HomeWiP’s ADU plans are often ready to permit. Every plan set includes the architectural drawings, easy to adapt site-specific structural engineering, and fast-track energy calculations required for permit submittal in California and most other states.
Your lot size and local height restrictions will push you toward one layout or the other. Here’s how they compare.
One-Story ADU Plan: The entire unit sits on the ground, so you need a larger footprint. A 750 sq ft one-story ADU requires a buildable zone of roughly 25 ft × 30 ft. The advantages: no stairs, fully ADA-adaptable for aging-in-place, simpler construction, and lower cost per square foot. Best for wide, flat lots with generous setbacks.
Two-Story ADU Plan: Half the ground footprint — a 750 sq ft unit needs only about 20 ft × 19 ft on the ground. Most two-story ADU floor plans place the living area and kitchen downstairs with bedrooms upstairs. The tradeoffs: stairs limit accessibility, structural requirements add cost, and a second-floor bathroom increases plumbing complexity. Best for narrow urban lots or when you want to preserve yard space.
Both configurations work within most ADU ordinances. California’s statewide ADU law permits accessory dwelling units up to 1,200 sq ft, and most cities allow two-story ADUs up to 16 feet in height (some up to 25 feet near transit). Check your city’s specific height limit before choosing a two-story plan.
Start with three measurements: your lot width, the setbacks your city requires (typically 4 ft from side and rear property lines for ADUs in California), and the buildable area that’s left after subtracting setbacks and the footprint of your primary residence.
Next, decide on the purpose. A rental unit benefits from a second bathroom and bedrooms with exterior windows. An aging-in-place unit needs wide doorways (36 inches minimum), a step-free entrance, and a ground-floor bathroom. A work-from-home layout might trade the second bedroom for a dedicated office with a separate entrance.
Then choose your size bracket based on what fits physically and what serves your goals. A backyard ADU plan for a 40-foot-wide lot will look very different from one on a 60-foot-wide lot.
HomeWiP’s plan pages include setback diagrams and city-specific notes to help you verify fit before you commit.
After choosing the right plan from a pre-approved ADU plans program or gallery, property owners should review their available space, consider setbacks, and prepare for the construction of ADUs by gathering documents such as a soils report, site plan, and plot plan.
If you need modifications for a specific site, partnering with a local ADU expert ensures compliance with all local regulations and codes. We are happy to help you get started with a site plan.
With permit-ready ADU plans and valuable input from city or county officials, your ADU project can transition smoothly from architectural plans to the construction phase, eliminating many of the traditional barriers to building secondary units.
Local ADU resource centers in Alameda, Santa Clara, and other California counties make it easy for property owners to browse a comprehensive gallery of house plans, floor plans, and pre-approved ADU building plans. These centers—sometimes called ADU accelerators—are designed to support efficient ADU development and help you get the most out of your available land.
Casita coalition, the best known non-profit in the ADU space, has a lot of useful resources for those who prefer their own research.
California has the most ADU-friendly laws in the country. Key provisions that affect your ADU plan selection: the statewide maximum is 1,200 sq ft for detached ADUs, cities must approve compliant ADU applications within 60 days, and most single-family lots are eligible regardless of local zoning.
Several counties run pre-approved plan programs that speed up permitting even further. HomeWiP participates in some programs in Alameda County and Santa Clara County, where pre-approved plans can reduce permit review from 60 days to as few as 2 weeks.
If you’re outside California, ADU regulations vary widely. Oregon, Washington, and several other states have passed ADU-friendly legislation, but setback rules, size limits, and permit timelines differ. Start with your city’s planning department website for a general overview.
Building an accessory dwelling unit is increasingly accessible thanks to ready, pre-approved plans, available resources, and supportive city and county programs across California. Whether you’re seeking an in-law suite, granny flat, or a backyard cottage for rental income, the option to use streamlined, pre-designed plans is the key to fast, affordable, and compliant ADU construction. Leverage the power of ADU centers, galleries, and resources in your community to take the first step toward your own successful ADU development project.
Read more: How to finance an ADU: 10 powerful options
