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Accessory Dwelling Units(ADUs) have moved from a niche project to a serious option for homeowners who want rental income, multigenerational living space, or a high-quality guest suite without leaving the lot they already own. The right ADU can take many forms, from a basement or garage conversion to an addition or a detached structure. Costs vary based on the type of ADU, size, utility connections, zoning requirements, and project scope. This page walks through the factors that drive cost, how different ADU options compare, what a Bunn & Sons project includes, and what to watch for when comparing bids. If you’d rather skip ahead and get a real number for your lot, reach out and we’ll come walk it with you.
ADU costs vary significantly depending on the type of project. Converting a basement or garage, adding onto an existing home, and building a detached ADU each come with different construction requirements, permitting considerations, and levels of investment.
In the Denver Metro area, many ADU projects start around $150,000-$200,000, with costs increasing based on size, site conditions, utility connections, and the level of finishes selected. Detached ADUs, in particular, often require additional investment for foundations, utilities, and site work.
ADU costs scale with square footage. Boulder caps market-rate detached ADUs at 800 square feet, and Broomfield restricts ADUs to 50 percent of the principal home’s footprint or 800 square feet, whichever is less. Smaller units are more affordable in absolute dollars, but cost-per-square-foot often rises on smaller builds because fixed costs (foundation, utility hookups, kitchen, bathroom) get spread across less space.
The right ADU starts with the right strategy. Sometimes the best solution is creating an independent living space within your home’s existing footprint through a basement or garage conversion. Other times, a detached ADU offers the privacy, functionality, and flexibility your family needs. We help homeowners evaluate the tradeoffs and choose the approach that best fits their goals, property, and investment.
A detached ADU is a small custom home in everything but name. Foundation work, framing, roof, and weather envelope all carry meaningful costs, and decisions like crawl space vs. slab, single-story vs. two-story, and roof complexity can swing the budget significantly.
Running new water, sewer, gas, and electrical service from the main house or street to the ADU is one of the most variable line items in any ADU project. Distance, trenching, depth of existing utilities, and whether you need a separate meter or upgraded service all impact cost.
Even a compact ADU usually includes a full kitchen and at least one full bathroom. Cabinetry, countertops, plumbing fixtures, tile, and appliances drive a larger share of total cost on a small unit than they do on a whole home, because the unit has so much less square footage to absorb them.
Boulder County and surrounding jurisdictions enforce strict energy codes that affect insulation, windows, mechanical systems, and air sealing. Meeting these codes properly often pushes ADUs toward heat pumps, high-performance windows, and tighter envelopes than older homes were built to. Done well, these systems are part of why a high-quality ADU rents and holds value.
Lot conditions matter. A flat lot with easy equipment access, existing utility stubs, and clear setbacks costs less than a sloped lot, a tight infill site, or a lot where existing landscaping, mature trees, or other structures have to be worked around.
Each jurisdiction in our service area has its own ADU rules, including height limits (Broomfield caps at 25 feet), parking requirements, and review timelines (Boulder is typically 40 to 60 days). We manage the permit process and stay current with state-level changes like HB24-1152.
The same square footage can cost two or three times more depending on finish level. Tile, flooring, lighting, hardware, plumbing fixtures, cabinetry, and millwork are where homeowner taste shows up most directly in the budget.
A unit you plan to rent out long-term, or live in yourself eventually, justifies a different finish level and durability standard than a guest house used a few weekends a year. We design and price differently depending on how you actually plan to use the space.
Getting multiple quotes for an ADU and finding a wide spread between them is completely normal, and it rarely means one contractor is simply more expensive. Here’s what actually drives the difference:
In-house oversight vs. loosely managed subcontractors
We work with vetted long-term trade partners rather than rotating the lowest bidder, which costs more upfront but produces a tighter, more predictable project. Contractors who pull together a different crew on every job often deliver lower bids and looser results.
Whether the contractor pulls permits and understands local ADU rules
Permit management is built into our process, including securing building permits during the PDA phase. We're particularly experienced with Broomfield, Boulder, Louisville, Lafayette, and Golden ADU code requirements, and we track state-level changes like HB24-1152. A bid that doesn't include permits, or assumes the homeowner navigates ADU zoning on their own, is not the same scope of work.
Custom detached build vs. prefab or garage conversion
Two ADU bids can be wildly different because they're pricing fundamentally different products. A custom detached unit, a prefab modular drop-in, and a garage conversion are three different projects, even though all three may be marketed as "ADU." Apples-to-apples comparison starts with comparing scope, not just price.
Fixed-cost vs. open-ended pricing
We commit to fixed-price contracts and explicitly reject time-and-materials. Our two-contract model uses a Project Development Agreement (PDA) to plan and price the project before we sign the build contract, so the number you sign is the number you pay outside of homeowner-driven changes. Open-ended bids almost always come in higher than they read on day one.
Whether design is fully resolved before pricing
In our PDA phase, design, engineering, selections, and trade partner input are all locked in before the build contract is written. Contractors who quote off rough sketches usually find "surprises" once construction starts. Those surprises become change orders.
Quality of materials specified
Two quotes can look very different because they're pricing different cabinets, different windows, different mechanical systems, and different fixtures. The cheaper bid may be specifying entry-level materials that won't hold up in a unit you plan to rent or live in long-term. We specify materials that match the investment and the longevity of the unit.
Every Bunn & Sons ADU build includes the following. There are no hidden line items and no surprises at contract signing.
Before we ever quote a fixed price, we run you through our PDA, a legally binding pre-construction phase that locks in design, engineering, selections, scope, schedule, and budget. This is what allows us to deliver a real fixed-price contract instead of a hopeful estimate.
Once the PDA is complete, we issue a fixed-price contract for the construction phase. The number you sign is the number you pay, outside of changes you choose to make during the build.
Design, engineering input, and selections are resolved during the PDA, not during construction.
You’ll know what work is happening when, who’s on site, and where we are against the timeline at any given point in the project.
We manage permit applications, timelines, and inspection coordination as part of the PDA and build phases. We’re particularly familiar with Broomfield, Boulder, Louisville, Lafayette, and Golden ADU rules and stay current with state-level changes.
We don’t rotate subs job to job. The plumbers, electricians, framers, and finish trades on your project have worked with us before and will work with us again, which is one of the reasons our outcomes are predictable.
We stand behind the work, and we’re still in business after the project ends to make good on it.
Tom, Brook, and our superintendent Jay run day-to-day execution, and ownership stays close to every project given that we only run up to three at a time.
What is not typically included: furniture, art, landscaping beyond what’s required by code, and any homeowner-supplied materials, unless they’re spelled out in the contract. We’ll tell you exactly what’s in scope and what’s not before you sign.
A lower bid from another contractor often looks attractive until you compare line items. If their number doesn’t include permits, full design, utility connections, finished selections, or a real schedule, you’re not comparing the same project. The “cheap” bid usually catches up by the end of construction.
A “$160,000 ADU” and a “$700,000 ADU” can both technically be called ADUs, but they’re radically different products. Prefab modular drops, garage conversions, basement conversions, and custom detached new builds all show up in the same data sets, which pulls the “average” toward whichever type happens to be most common in a given market. Aggregated figures published for our area are useful directional data, but they don’t tell you what a custom, code-compliant detached ADU built to long-term standards will actually cost.
Stringent local energy codes, demand for sustainable and high-end finishes, longer permit timelines, and a competitive trade labor market all push pricing up locally. National ADU calculators almost always understate what a real detached project here will cost, especially once utility connections, design, and code compliance are properly priced in.
A bid that comes in well below the rest is usually missing something. The most common omissions on a low ADU bid are: utility trenching and connections, structural engineering, full kitchen and bath finish selections, energy-code compliance work, design, and permit fees. The result is a “starting price” that grows steadily through change orders. By the end of construction, the cheap bid is rarely cheap. Comparing a complete fixed-price scope to a partial open-ended estimate isn’t a fair comparison, even if both numbers technically describe an “ADU.”
Investment: $400k
When Patricia faced the future alone, her son Ryan and his family opened their home to her. What started as an unfinished 1,600-square-foot basement became a self-sufficient living suite designed around Patricia’s needs, her style, and the quiet independence she wanted to hold onto.
Timeline: 4 Months
An ADU is a significant investment, and the right decision depends as much on how you plan to use it as on the math. Here’s how we think about it.
You want long-term rental income on a lot you already own.
A well-built detached ADU in our area can rent strongly given the Boulder and Broomfield housing market. Over a long enough hold, that rent often covers a meaningful portion of the build cost.
You're planning for multigenerational living.
Aging parents, adult children, or extended family who want privacy and independence on the same property are one of the strongest use cases for a detached ADU. It's a long-term solution, not a workaround.
You want a home office, studio, or guest space without remodeling the main house.
An ADU keeps work, hobbies, or guests out of the primary home and adds a separate, fully self-contained structure on the lot.
You're planning to stay in the home long-term.
ADUs pay back over time. The longer you stay, the better the math gets, both financially and in terms of how much you actually use the space.
Your lot can support it.
Lots with usable yard space, reasonable access for construction equipment, and clear setbacks make ADUs significantly easier and less expensive to build.
You value certainty of outcome.
If you want a contractor who quotes a real fixed price, manages permits and ADU zoning, runs the project, and stands behind the warranty, a design-build firm is built for that. If price is the only variable that matters, a high-volume general contractor or prefab vendor will likely come in lower.
You just need extra storage or a workshop.
A detached ADU is overbuilt for storage. A simple shed, garage, or workshop will get you what you need at a fraction of the cost.
You're planning to sell within 2 to 3 years.
ADUs add resale value, but you typically won't recover the full investment over a short hold. If you're moving soon, the math doesn't work the same way.
Your lot is too small or too constrained.
Setbacks, height limits, and lot coverage rules can make an ADU impractical or impossible on certain properties. The lot has to actually support the unit you want.
You're looking for the lowest possible price.
The lowest proposal isn't always the best value. A lower price for an ADU definitely means something's been left out, less planning, fewer services, lower quality materials, or scope items that surface later as change orders. We believe in transparent pricing and helping homeowners make informed decisions based on value, not just cost.
You haven't decided how you'll actually use the unit.
Designing a true rental, a long-term in-law suite, and a part-time guest house produces three different ADUs. Coming in with a clear use case is one of the most important things you can do before pricing the project.
Not every low estimate is a good deal. Here’s what to look for before you sign anything.
No permit pulled
ADUs trigger structural, electrical, plumbing, mechanical, and energy-code permits in our area, plus zoning review. A contractor who tells you permits aren't necessary, or quietly leaves them out of the bid to come in lower, exposes you to forced rework, fines, and serious problems at resale or when a tenant tries to move in. If permits aren't itemized in writing, they aren't being pulled.
No detailed walk-through before pricing
Whole-home remodels involve countless of decisions and existing conditions that can't be fully understood during a single visit. A thoughtful planning process helps uncover challenges, define scope, and align expectations before construction begins. Skipping this step often increases the likelihood of budget surprises and project changes down the road.
Vague scope of work
A real ADU proposal runs many pages and itemizes finishes, fixtures, allowances, structural components, utility connections, and inclusions room by room. A two-page proposal with a single lump-sum number and a generic scope description is not a contract, it's a marketing document. Vague scopes are where contractors hide change orders and where homeowner-contractor disputes are born.
No design or pre-construction process
On an ADU, skipping design and pre-construction is the single largest predictor of cost overruns. Without a structured planning phase like our PDA, decisions about layout, materials, utility routing, and finishes get made under pressure mid-construction, often at premium prices. If a contractor goes straight from "ballpark estimate" to "let's start digging," you should expect the final number to look very different from the starting one.
Most detached ADUs in Broomfield, Boulder, Louisville, Lafayette, and the surrounding area start around $200,000. The right number for your lot depends on size, site conditions, utility access, and finish level, which is exactly what our PDA phase is designed to nail down.
Yes, permit management is part of how we run our projects, including securing building permits and managing ADU zoning review during the pre-construction phase. We're particularly familiar with Broomfield, Boulder, Louisville, Lafayette, and Golden ADU permit timelines and code requirements, and we stay current with state-level changes like HB24-1152.
Our ADU projects typically run 6 to 11 months from start to completion, depending on size, site conditions, permit timelines, and finish complexity. We build a detailed schedule during the PDA phase and communicate it through Gantt charts during construction so you always know where the project stands.
We specialize in high-quality detached ADUs: full custom new builds with their own foundation, framing, kitchen, and bathroom. We don't do prefab modular drops or commodity garage conversions.
Local rules vary by jurisdiction, but in general, ADUs in our service area can be used as long-term rentals, in-law suites, guest houses, home offices, or owner-occupied primary dwellings. Boulder eliminated owner-occupancy and parking requirements as of March 2025, which has opened up more flexibility. We walk through your specific use case and your jurisdiction's rules during the PDA.
We use a two-contract model. First, a Project Development Agreement (PDA) covers the planning phase, where we resolve design, engineering, selections, scope, schedule, and budget. Once the PDA is complete, we issue a fixed-price build contract. There are no time-and-materials surprises and no open-ended numbers.
We surface as much as possible during the PDA, including site conditions, utility constraints, and likely zoning issues. If something genuinely unforeseen comes up during construction, we communicate it immediately, present options, and document any change order in writing before work proceeds. This is where the PDA pays for itself.
Furniture, art, landscaping beyond what's required by code, and homeowner-supplied materials are typically excluded unless we've specifically scoped them in. We're explicit about every inclusion and exclusion in writing before you sign the build contract.
Bunn & Sons
If you’re thinking about adding rental income, in-law space, or a flexible second dwelling on your lot and want to talk through what a real project would look like, we’d like to hear from you.