ADU Construction Cost Calculator

In 2026, building an ADU (accessory dwelling unit) typically costs $150 to $400 per square foot, so a common 600-to-800-square-foot ADU runs roughly $100,000 to $300,000+ all-in, depending on the type and finishes. A garage conversion is the most affordable option (often $80,000 to $150,000) since the structure already exists, while a detached new-build ADU is the most expensive (often $200,000 to $400,000+) because it's a complete small home with its own foundation, walls, roof, kitchen, bathroom, and utility connections. The main cost factors are the ADU's size, the type (garage conversion, attached, detached, or prefab), the finish level, the site conditions, and the cost of utilities, permitting, and design. ADUs cost more per square foot than a full house because their small size means fixed costs (kitchen, bath, utilities, permitting) are spread over less area. Despite the cost, ADUs are popular for generating rental income, housing family members (in-law or 'granny' flats), increasing property value, and adding flexible living space. This calculator lets you adjust the size, type, finish, and site conditions to estimate your ADU construction cost.

An ADU (Accessory Dwelling Unit) is a secondary, self-contained living unit on the same lot as a primary home — it has its own kitchen, bathroom, sleeping, and living space, and a separate entrance. ADUs are also called granny flats, in-law suites, backyard cottages, casitas, or secondary units. There are several types: A detached ADU is a standalone structure separate from the main house (like a backyard cottage), offering the most privacy and flexibility but costing the most to build. An attached ADU is built as an addition connected to the existing house, sharing a wall. A garage conversion turns an existing garage (attached or detached) into living space, which is the most economical since the basic structure exists. An above-garage or garage-apartment ADU adds living space over a garage. A prefab/modular ADU is built in a factory and delivered and set on site, which can be faster and sometimes more cost-predictable. A basement or interior conversion creates an ADU within the existing home's footprint (like a basement apartment). The type you choose depends on your property, budget, local regulations, and how you'll use the ADU. Many areas have relaxed ADU rules to encourage housing, but zoning, size limits, setbacks, and permitting vary widely by location. This calculator covers garage-conversion, attached, detached, and prefab ADU types.

ADUs often cost as much as or more per square foot than building a full-size house, which surprises many people, but there are clear reasons. The biggest factor is that an ADU is a small, complete dwelling — it needs everything a house needs (a kitchen, a bathroom, plumbing, electrical, HVAC, insulation, a foundation, walls, a roof, windows, doors, and finishes) but packed into a small footprint, so those fixed costs are spread over far fewer square feet, raising the per-square-foot rate. A full kitchen and bathroom alone are expensive and represent a much larger share of a 600-square-foot ADU than of a 2,500-square-foot house. Other cost drivers: connecting utilities (water, sewer, electrical, sometimes gas) to a separate or detached unit can be expensive, especially if lines must be run across the property or the electrical panel upgraded; permitting, design, and impact fees add up and aren't proportionally smaller for a small unit; site work, access challenges, and working in an existing backyard add cost; and economies of scale that benefit larger builds don't apply. Garage conversions are cheaper per square foot because the shell exists, but new detached ADUs carry the full per-foot cost of a complete tiny home. So while the total ADU cost is much less than a full house, the per-square-foot cost is high. This calculator reflects these realities with type-based rates and utility/permit add-ons.

Yes, converting an existing garage into an ADU is generally the most economical way to create an ADU, often costing significantly less than building a new detached unit. The reason is that a garage conversion starts with an existing structure — the foundation/slab, walls, and roof are already there — so you avoid the cost of building the shell from scratch, which is a major portion of new construction. The conversion work focuses on making the space livable: adding insulation, drywall, flooring, windows and an entry door, plumbing for a kitchen and bathroom, electrical, HVAC, and finishes. A garage conversion might run $80,000 to $150,000 versus $200,000+ for a comparable detached new-build. However, there are considerations: the existing garage's size limits the ADU size, the slab and structure may need upgrades to meet residential code (insulation, ceiling height, moisture/foundation issues, egress windows), you lose the garage/parking (and some areas require replacement parking), and connecting utilities is still needed. Garage conversions work best when you have a suitable garage you're willing to give up and want the most cost-effective ADU. If you need more space, more privacy, or don't want to lose the garage, a detached or attached ADU may be worth the higher cost. This calculator's garage-conversion option reflects the lower cost of using an existing structure.

Yes, building an ADU almost always requires permits and must comply with local zoning and building codes, and the regulations are a significant part of the process and cost. You'll typically need building permits, and the project must meet zoning rules (which govern whether ADUs are allowed, the maximum size, setbacks from property lines, height limits, parking requirements, and owner-occupancy rules) and building codes (for structural, electrical, plumbing, insulation, egress, fire safety, and more). Many states and cities have recently relaxed ADU regulations to encourage more housing — streamlining approvals, reducing parking requirements, and allowing ADUs in more areas — but rules still vary widely by location, so checking your local requirements early is essential. The permitting and approval process can take weeks to months and involves submitting plans (often requiring professional design/architectural drawings), paying permit and sometimes impact fees, and passing inspections during construction. Utility connections may need separate approvals. Because of this complexity, design and permitting is a real cost and timeline factor (this calculator offers a design-and-permitting add-on), and many homeowners work with ADU-experienced contractors or designers who know the local process. Don't skip permits — an unpermitted ADU can't be legally rented, causes problems at resale, and may have to be removed or retroactively permitted. Research your local ADU rules before planning. This calculator includes a design-and-permitting option.

Prefab (factory-built/modular) ADUs offer potential advantages in speed and cost predictability, though they're not always dramatically cheaper than site-built, and there are trade-offs. Speed: prefab ADUs are built in a factory while site work (foundation, utilities) happens in parallel, then the modules are delivered and set, so the on-site construction time is much shorter — the unit can be installed in days once the site is ready, versus months of stick-building. This faster timeline is a major appeal. Cost: factory construction can offer efficiencies and more predictable pricing (less weather delay, bulk materials, streamlined labor), and prefab can be competitive, but the unit cost plus delivery, crane/set, foundation, and on-site utility hookups and finishing means prefab isn't always cheaper than a comparable site-built ADU — sometimes it's similar. The savings depend on the product and your site. Trade-offs: prefab ADUs come in set models/sizes with less customization than a fully custom site-built unit, delivery requires adequate site access for trucks and a crane, and you still need a foundation, utilities, and permits. Prefab is appealing for its speed, predictability, and reduced on-site disruption, and works well on accessible sites; custom site-built offers more flexibility for unusual lots or specific designs. This calculator includes a prefab/modular ADU type. Weigh speed and predictability against customization and site access when deciding.

Yes, an ADU can add significant value and income to your property, which is a primary reason homeowners build them — though the return depends on your local market and how you use it. Rental income: an ADU can be rented out (long-term or, where allowed, short-term) to generate ongoing income, which can offset the construction cost over time and provide cash flow — in high-rent areas, an ADU can pay for itself in several years. Property value: a well-built, permitted ADU typically increases your property's value, since it adds livable square footage and income potential that appeals to buyers; the value boost varies by market but can be substantial, especially where housing is scarce and ADUs are in demand. Flexible use: beyond rental income, ADUs provide valuable space for aging parents (in-law suites), adult children, guests, a home office, or as a place to live while renting out the main house — this flexibility has its own value. Considerations: the return depends on local rents and home values, whether the ADU is permitted (unpermitted units don't add legitimate value and can't be legally rented), and your costs; in some markets the value added may not fully equal the build cost, while in high-demand areas it can exceed it. ADUs are generally considered a sound investment in areas with strong rental demand and supportive regulations. This calculator helps you estimate the construction cost to weigh against the potential income and value. Consult local real estate and rental data for your specific return.

Building an ADU typically takes several months, with the timeline varying widely by type and including a often-lengthy design and permitting phase before construction even begins. A rough breakdown: Design and permitting can take 1 to 6 months (or more) depending on your jurisdiction — creating plans, submitting for permits, and getting approvals is frequently the slowest part, especially in areas with backlogs (though some places have streamlined ADU approvals). Construction time then depends on the type: a garage conversion is the fastest (often 2 to 4 months) since the structure exists; an attached or detached site-built ADU typically takes 4 to 8 months to construct; and a prefab/modular ADU has a short on-site installation (days to a few weeks once the site and foundation are ready) but the factory build and site prep add time, often totaling a few months. So from start (design) to a finished, move-in-ready ADU, expect roughly 6 to 12 months for most projects, with garage conversions and prefab potentially faster and complex custom detached units longer. Factors that extend the timeline include permitting delays, utility connection complications, site challenges, weather, contractor availability, and change orders. Starting the design and permitting process early and working with ADU-experienced professionals helps keep things moving. This calculator estimates the cost; the timeline depends on the type, your local permitting, and site conditions.