TL;DR: Twin Falls ADU Guys offers six floor plans ranging from a 450 square foot studio to a 1,200 square foot two-story unit. The right plan depends on three things: what your lot can physically fit (setbacks, lot coverage, and the 45 percent size cap in Twin Falls zoning), how you plan to use the unit (rental income, family housing, home office), and what your budget allows. This guide walks through each factor and matches our six plans to the most common use cases so you can narrow your options before your first design conversation.
The 45 Percent Rule: Start Here Before You Fall in Love with a Floor Plan
Before you browse layouts and pick finishes, there is one local rule that narrows the field immediately. The Twin Falls Uniform Development Code caps ADU size at 45 percent of your primary home's living area. That means the maximum ADU you can build is determined by the size of the house already on your lot.
Here is how that math works for common Twin Falls home sizes.
A 1,200 square foot home allows an ADU up to 540 square feet. A 1,500 square foot home allows up to 675 square feet. A 1,800 square foot home allows up to 810 square feet. A 2,000 square foot home allows up to 900 square feet. A 2,400 square foot home allows up to 1,080 square feet. And a 2,700 square foot home or larger allows up to 1,200 square feet, which is the largest plan currently available.
This is the single most important number in your floor plan decision. A homeowner with a 1,400 square foot ranch home cannot build a 900 square foot two-bedroom unit regardless of lot size or budget. The zoning caps it at 630 square feet. Knowing your maximum before you start comparing plans saves time and avoids disappointment.
If you are unsure of your home's exact living area, your county assessor records or your original appraisal will have the number. The Twin Falls County GIS system is a good starting point for parcel data.
What Your Lot Actually Allows
The 45 percent rule tells you the maximum size. Your lot tells you the maximum footprint, and those are not the same thing.
Every lot in Twin Falls has required setbacks from property lines, and your ADU must sit within whatever buildable area remains after those setbacks are subtracted. Twin Falls zoning also requires your ADU to match the primary home's design: roof pitch, siding material, and window style all need to be consistent. One additional parking space is required, and only one ADU is allowed per lot.
Corner lots are especially tricky. Two sides face public streets, which often means stricter setback requirements on both frontages. A lot that looks spacious from the curb may have a surprisingly small buildable zone once the rules are applied. We cover how setbacks and lot constraints affect ADU feasibility in detail in a separate guide.
For homeowners on smaller parcels in communities like Kimberly or Filer, lot constraints can be even tighter. Our small-lot ADU guide covers strategies for making the most of limited space.
The key point: do not pick a floor plan and then check whether it fits your lot. Check your lot first, then pick from the plans that actually work.
Six Plans, Six Different Situations
Each of our floor plans is designed for a specific combination of lot size, use case, and budget. Here is what each one is built for and who it works best for.
The Perrine: 450 square feet, studio, one bath, detached. This is the most compact option and the easiest to fit on constrained lots. It works well as a rental unit targeting single tenants or traveling professionals, or as a private home office with a full bathroom. Because it is a studio layout, it maximizes usable space without hallways or wasted square footage. It fits under the 45 percent cap for any primary home over 1,000 square feet. If your lot is tight on setbacks or your home is on the smaller side, this is likely your starting point.
The Shoshone: 600 square feet, one bedroom, one bath, garage conversion. This is the only plan designed specifically as a garage conversion rather than a new detached structure. Converting an existing garage eliminates foundation work, reduces permitting complexity, and typically costs significantly less than new construction. It is a strong option for homeowners who have a two-car garage they are willing to repurpose. The separate bedroom makes it more attractive to long-term renters than a studio layout. Your primary home needs to be at least 1,334 square feet to clear the 45 percent threshold. For a deeper comparison of conversion versus new construction, our garage conversion guide covers the tradeoffs.
The Snake River: 750 square feet, one bedroom, one bath, detached. This plan offers more breathing room than the Perrine while still fitting comfortably on most standard residential lots. The additional square footage goes into a larger living area and a more generous kitchen layout. It works well for aging parents who need an independent living space close to family, or for long-term rental tenants who want a one-bedroom with room to spread out. Your primary home needs to be at least 1,667 square feet.
The Magic Valley: 900 square feet, two bedrooms, one bath, detached. This is where the floor plans shift from one-bedroom to two-bedroom layouts. The second bedroom opens up new use cases: a couple with a child, a tenant who works from home and needs a dedicated office, or a family member who occasionally hosts overnight guests. Two-bedroom units also command higher rents. Current Twin Falls rental data shows that the jump from one bedroom to two bedrooms adds meaningful monthly income. Your primary home needs to be at least 2,000 square feet.
The Sawtooth: 1,050 square feet, two bedrooms, two baths, detached. The second bathroom is the differentiator here. For aging parents, a second bath is not a luxury but a practical necessity, especially if mobility is a consideration. For rental income, a two-bed, two-bath unit competes directly with small apartments rather than just accessory units, which supports stronger rental rates. This plan requires a primary home of at least 2,334 square feet.
The Twin Falls: 1,200 square feet, two bedrooms, two baths, two-story, detached. This is the largest plan and the only two-story option. The vertical design means a smaller ground-level footprint, which can actually make it easier to fit on lots where horizontal space is limited but the 45 percent cap allows the square footage. It is the closest to a standalone small home in terms of livability and rental appeal. Your primary home needs to be at least 2,667 square feet to qualify.
Matching Your Plan to Your Use Case
The right floor plan depends less on what you like and more on what you are trying to accomplish. Here is how the most common use cases map to specific plans.
Maximizing rental income. One-bedroom units (the Perrine, Shoshone, or Snake River) offer the best return per square foot of construction because they cost less to build while still commanding competitive rents. If your goal is cash flow and your lot supports it, a well-built one-bedroom unit often makes more financial sense than stretching for a two-bedroom. We break down how rental income and financing work for ADU projects in a separate guide.
Housing an aging parent. The Snake River (750 square feet, one bed, one bath) or the Sawtooth (1,050 square feet, two bed, two bath) are the strongest options here. The key considerations are single-floor living, bathroom accessibility, and enough space to feel like a real home rather than a guest room. If your parent occasionally has a caregiver or overnight visitor, the second bedroom in the Sawtooth is worth the additional investment.
Housing a young adult child. The Perrine (450 square feet studio) or the Shoshone (600 square feet garage conversion) both work well. Young adults typically need less space and value independence more than square footage. A garage conversion is especially practical here because it keeps costs down while giving your child a separate entrance and their own address.
Home office or creative studio. The Perrine is purpose-built for this. A 450 square foot detached studio with a full bathroom gives you a professional workspace that is physically separated from your home. That separation matters for productivity, and if you ever decide to convert it to a rental unit later, the layout already works.
Guest house with future rental potential. The Magic Valley (900 square feet, two bed, one bath) balances hospitality with long-term flexibility. Two bedrooms accommodate guests comfortably, and the layout converts to a rental unit without modification when you are ready to generate income.
The Budget Factor
Floor plan choice and budget are connected, but not in a straight line. Larger units cost more in total, but the cost per square foot often decreases as size increases because fixed costs like permitting, utility connections, and site preparation are spread across more living area.
Garage conversions (the Shoshone) are typically the most cost-effective path because they reuse an existing foundation and building envelope. New detached units cost more but offer greater flexibility in placement and design. Two-story construction (the Twin Falls plan) can reduce foundation and roofing costs per square foot compared to a single-story unit of the same size.
For a realistic picture of what each type costs in the Twin Falls market, our April build timeline guide includes current local cost ranges for conversions, detached builds, and pre-fab options.
Quick Reference
The Perrine: 450 square feet, studio, one bath, detached. Minimum primary home: 1,000 square feet. Best for: rental income, home office.
The Shoshone: 600 square feet, one bedroom, one bath, garage conversion. Minimum primary home: 1,334 square feet. Best for: young adult housing, budget-conscious rental.
The Snake River: 750 square feet, one bedroom, one bath, detached. Minimum primary home: 1,667 square feet. Best for: aging parent, long-term rental.
The Magic Valley: 900 square feet, two bedrooms, one bath, detached. Minimum primary home: 2,000 square feet. Best for: guest house, family rental.
The Sawtooth: 1,050 square feet, two bedrooms, two baths, detached. Minimum primary home: 2,334 square feet. Best for: aging parent needing accessibility, premium rental.
The Twin Falls: 1,200 square feet, two bedrooms, two baths, two-story, detached. Minimum primary home: 2,667 square feet. Best for: maximum livability on a constrained footprint.
Ready to Find Out Which Plan Fits Your Property?
If you have a plan in mind or are still deciding between options, the fastest way to narrow it down is a conversation with someone who knows your local zoning and lot constraints. Twin Falls ADU Guys offers a free Readiness Call that takes 10 to 15 minutes. We will ask about your property, your home size, and your goals, then tell you which of our six plans are realistic options for your lot. If none of them fit as designed, we will explain why and discuss what modifications might work.
We serve homeowners across Twin Falls, Jerome, Burley, Rupert, Filer, Buhl, Kimberly, Gooding, and Hagerman. You can schedule your Readiness Call at twinfallsaduguys.com or call us directly at (208) 613-9830.
Frequently Asked Questions
Can I modify one of your floor plans to fit my lot?
Yes. Our six plans are starting points, not rigid templates. Modifications to dimensions, layout, and features are common and expected. The key constraint is that any modified plan still needs to meet Twin Falls zoning requirements, including the 45 percent size cap, setback rules, and design match standards. A feasibility check confirms what modifications are possible on your specific lot.
What if my home is too small for the plan I want?
The 45 percent rule is a hard cap set by Twin Falls zoning. If your primary home is 1,500 square feet, your maximum ADU size is 675 square feet regardless of lot size or budget. In that scenario, the Perrine (450 square feet) or the Shoshone (600 square feet) would be your options. The good news is that smaller units often deliver strong returns because they cost less to build while still commanding competitive rents.
Is a garage conversion always cheaper than a detached unit?
In most cases, yes. Conversions reuse the existing foundation and building envelope, which eliminates two of the most expensive phases of new construction. However, some garages need significant structural work, electrical upgrades, or foundation reinforcement that can close the cost gap. A site assessment will tell you whether your garage is a strong conversion candidate.
Which plan generates the most rental income?
Two-bedroom units command higher monthly rents, but one-bedroom units often produce better returns relative to construction cost. The answer depends on your total investment, your financing structure, and local demand in your specific neighborhood. Current Twin Falls market data supports strong demand for both one-bedroom and two-bedroom rentals.
Do I need to match my ADU's exterior to my existing home?
Yes. Twin Falls zoning requires that ADUs match the primary home's roof pitch, siding material, and window style. This requirement applies to all six plans. Your designer will work with these constraints during the plan development phase to ensure compliance without sacrificing the interior layout you want.
Can I build a two-story ADU on my lot?
Possibly. The Twin Falls plan is our two-story option, and its smaller ground-level footprint can actually make it easier to fit within setback requirements. However, two-story construction may trigger additional review depending on your zoning district, and height limits vary. Confirm with the Twin Falls Planning and Zoning office at (208) 735-7267 before committing to a two-story design.
Twin Falls ADU Guys Team
Twin Falls ADU Guys
