How Long Does It Actually Take to Build a Detached ADU?

By Buz Artiano, BuildX | April 2026

Watch: Lt. Governor Kim Driscoll: How ADUs Are Solving Massachusetts' Housing Crisis One Family at a Time

You've heard that building an ADU could take a year or longer. There's been stories about just the permits taking months to get, contractors disappearing after collecting large amounts of money, and budgets getting out of control because the project just kept taking longer and longer. If you're trying to decide whether to build a detached ADU for a parent, an adult child, or rental income, the timing isn't just about the timing, time costs money too. Every extra month the project takes means you're paying for two homes for another month and all that extra waiting time is making you wonder if this was even the right decision.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need, we handle design, permitting, and construction, which is how we can control the timeline of your project. So we see every phase of a project from the first site visit up until you move in. That's how we can give you a straight answer on how long this actually takes.

The honest answer is that most of what you've read about how long it takes to build an ADU is either way out of date or very vague. The industry norm of quoting "six to twelve months" really doesn't tell you anything. In this market, with the right builder and the right process, a detached ADU can go from digging the foundation to being move-in ready in about 100 days. We're going to explain what you need to know about that number, what sort of things can make it take longer, and how to decide whether a builder can live up to his timeline claims.

How Long Does the Construction Phase Actually Take?

We guarantee a 100-day build from the day we start digging. That's for a fully detached, stick-built new construction ADU on a residential lot in Massachusetts. It's a custom build, not a prefab modular home. It has a full foundation, framing, mechanical systems, finishes, and final inspections.

The longest build we've ever done took 115 days. That project had some difficult site conditions and a very complex septic installation. We had evaluated all that ahead of time, so we made sure we told the homeowner before they signed. The 100-day guarantee isn't a gimmick we use to get sales. It's a commitment we make because we can back it up with our honed process and the way we manage our subcontractors.

When Massachusetts Lt. Governor Kim Driscoll visited our operation and heard the 100-day timeline, she told us that she wished all construction projects could say the same thing. That's not an endorsement, it's just a reaction from someone who oversees housing policy for the entire state and knows what construction timelines usually look like.

To put that in perspective, the industry norm for a custom-built detached ADU is somewhere between six and twelve months, but some builders take even longer. That's a huge range, which should make you wonder why the builder doesn't control enough of their own process to give you a more solid number.

What Happens Before the 100-Day Clock Starts?

The 100-day guarantee covers construction, from excavation up to being move-in ready. But the full timeline from your first phone call to move-in day is longer than that. Before any construction can even start, several other things need to happen, and each one takes a different amount of time.

Design typically takes two to four weeks, depending on how quickly you finalize your floor plan, finish selections, and any site-specific changes. Because we handle design in-house, there's no going back and forth between your architect and your builder. Just that can save weeks compared to how it's usually done where you would hire an architect, get plans drawn, then take those plans to interview different builders.

Permitting varies by municipality. Under the Massachusetts Affordable Homes Act, ADUs are by-right, which means you pull a building permit without needing a special permit or zoning variance. We've found that most towns will process a building permit in two to six weeks. Some are faster, some are slower, it just depends on the town, especially if the building department is backlogged or if you also need a septic review.

Financing and the analysis of the septic system are done at the same time with the design and getting permits for most people. Septic is the one thing that changes and that can catch some people off guard. If your lot must have a new or upgraded septic system, testing the soil, a review by the Board of Health and engineering will all add extra time before the actual construction can begin. We investigate the condition of the septic system early so there are no surprises after construction begins.

The total pre-construction phase, from first conversation through pulling the building permit, usually takes two to four months. Combined with the 100-day construction window, most families should plan for a total timeline of roughly six to eight months from first call to move-in. That is a realistic, honest number.

What Can Push the Timeline Past 100 Days?

Not every build finishes in exactly 100 days. Which we have been honest about from the beginning. Our longest project ever took us 115 days, and we told the homeowner before we signed the contract that their project would take slightly longer than our normal 100-day guarantee.

The variables that affect the construction timeline are easy to predict and plan for. We go over and evaluate all of them before we quote, not after we start building.

Septic complexity is the single most common factor on any build. If we're able to do a simple tie-in because your system has extra capacity, that adds the least amount of time. A failed perc test that requires a specially engineered septic solution can add weeks to the schedule, because the whole system needs to be installed and inspected before we can pour the foundation.

Site conditions make a big difference. Steep grades, no easy access for equipment, ledge, or high water tables all affect excavation and foundation work. On a flat, clear lot with good soil, we move fast. On a lot with constraints, we plan for them and let you know how much the timeline will need to be adjusted.

Weather is the one that nobody controls. An extended period of heavy rain when we're trying to dig the basement, or a long freeze during foundation work can push things back by as much as a couple weeks. We account for that when we plan, but extreme weather events are just something nobody can control.

But with us, your timeline will never get pushed back because of bad project management, material delays because of last-minute ordering, or subcontractors who don't show up. Those things are simple failures in the planning process, they're not variables. The reason our builds finish on time is that we control the entire process from design through final walkthrough.

Why Do Most Builders Quote Six to Twelve Months?

The wide timelines you see from other builders are not just conservative estimates. They reflect a fundamentally different way of running a project.

Most residential builders have a process that's split up and doesn't flow smoothly. The homeowner hires an architect to produce plans. Then they shop those plans to multiple builders and each builder gives them a quote based on their own interpretation of the drawings. After that the homeowner chooses their builder and that builder orders materials, lines up subcontractors, and begins work. All those different handoffs create delays and stretch the schedule out even further while running the risk of contractors not showing up on time or at all.

We get rid of all those handoffs. When we design the ADU, we have our own framing crews, mechanical subcontractors, and material suppliers. We know exactly what we're building before we even apply for the permit. That means we order all the materials during the permitting phase, not in the middle of construction. Our subcontractors are scheduled before the foundation is poured, not later on in the process. Doing things with this method and in this order is the difference between a 100-day build and one that can take a year. We don't have to work faster, we just don't stop working for random delays that could have been prevented.

When a builder quotes you a wide price range, ask them why they can't narrow down to be more specific. If it has to do with using outside architects and the availability of subcontractors, they have not honed their process to finish projects quickly. The way they do things is often designed for flexibility, for people who want to change their minds part way through, but doing it that way adds a lot of time to the build.

Does a Garage Conversion or Attached ADU Take Less Time?

If you already have a structure on your property that's worth converting, a garage or basement conversion can be faster than doing a separate structure as a new build. You don't have to do any excavation, foundation work, and you skip a lot of the framing. The Lt. Governor noted that converting an existing space can move a lot faster when the bones are still in good shape.

We've looked at garage conversions by other builders that moved quickly when the existing structure was up to code. The thing you have to decide is if it's worth saving. If the garage needs a new foundation, new roof, or a lot of structural reinforcement, that cuts right into the time you were saving in the first place. And sometimes it's faster and cheaper to build new than to try and fix up an old structure that wasn't really designed to be lived in.

An attached ADU, built as an addition to the main house, is usually somewhere between a conversion and a detached ADU as far as how long it takes. The shared wall does simplify some things, but it also means integrating with the main home's existing systems, which can complicate things a bit.

Our specialty is detached ADUs, and our 100-day guarantee applies to that. If a conversion or attached addition is the best option for your situation, we'll tell you that when we first speak with you, even if it means referring you to a different builder who specializes in those types of ADUs.

When Is BuildX Not the Right Fit for Your Timeline?

We are a design-build firm that specializes in detached, stick-built ADUs. That's what our 100-day guarantee covers. If your project is a garage conversion, an interior renovation, or a modular unit, we're not the right builder for you. We'd rather tell you that upfront than take on a project outside of our norm and run the risk of delivering something that isn't up to our standards.

If your timeline is pressured because of an emergency situation, such as a parent who needs housing next month, 100 days of construction plus two to four months of planning may not be fast enough for you. In those cases, a modular or prefab ADU, or temporary housing while construction is in progress, might be a better decision in the short term.

We also encourage every family to talk to more than one builder. If another builder tells you they can build something similar to what we can in less time, have them guarantee it in writing the way we do. A guarantee tells you something about how a builder manages their process. A vague estimate without a lot of detail and wide range timeline tells you they're just hoping for the best.

What Should Your Next Step Be If Timeline Matters to You?

If you're trying to decide whether to build a detached ADU in Massachusetts, the timeline is one of the most important factors in your decision. Every month of construction is a month you're paying for two households, managing a disrupted property, and waiting to use a space your family needs. A builder who can give you a firm, guaranteed construction timeline is not just saving you time. They are giving you the ability to plan the rest of your life around a number you can trust.