Does a slower ADU builder cost you more in loan payments?

By Buz Artiano, BuildX | July 2026

Watch: ADU Financing: How Families Build Together Without Buying Another House

You are looking at two different builders for the same ADU, and one of them is a few thousand dollars cheaper. So on paper, the choice seems obvious: choose the cheapest one and save the money. That's the thought just about every homeowner has when the price quotes are being looked at. And it's the same moment where the wrong builder can cost you money that doesn't show up on either estimate.

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 do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We evaluate septic constraints, zoning issues, utility connections and the requirement to get a permit from the city. We see something that's not usually talked about: the price of the quote is only part of what you pay.

Here's the part that gets missed. When you finance an ADU the way most families do, you are given the entire amount of the loan at closing and you start paying interest on all of it right away. That's even before your builder has used any of it on the finished ADU. That makes the time it takes to finish the construction part of the cost, one that never shows up on a quote. A builder who takes longer is also keeping your borrowed money longer, and you pay for every month of it. If you figure a cost on that time the cheaper bid often stops being the cheaper one. A slower build time also delays the time when your ADU starts paying you back, whether that's rent, a housing payment you combine with another one or just a parent who's moved in. Both costs deal with the same thing.

Your Loan Starts the Day You Close, Not the Day You Break Ground

Yes, a slower builder can cost you more, and the reason is in the detail most families don't think about. Most ADU builds are paid for with money that's pulled from your home. The common ways to finance are a cash-out refinance if your house is paid off or a renovation loan if you are borrowing against the property's future value. Both of them have one feature in common and should change how you think about time. There's no construction draw schedule so you don't get the money in pieces as the work goes to each phase. You get the entire amount at closing and it all goes into your account.

From that first day, you owe principal and interest on the whole balance. Principal is the loan itself and Interest is the bank's charge for lending you the money. You have to pay both on the whole loan even though most of that cash is still in your checking account waiting for the builder to need it. The difference between when the time starts and when money gets used is at the front. As we explain it to families, you don't need all of the money in the first 60 days, and the builder only takes parts of it as the ADU is being built, but you are paying interest already on every dollar you borrowed.

This is what makes a slow builder expensive. The lender doesn't care how fast your ADU is built because your payment is the same in the first month whether the foundation is poured or the site is still empty. So every extra month your builder takes is another full month that you are paying interest on money that's not finished becoming a building. Time is a bill you have to pay on a schedule.

How Much Does Each Extra Month Actually Cost?

The cost of time depends on three things: how much you borrowed, your interest rate, and how many extra months it takes to build the ADU. To think of it in real terms, take a $400,000 loan on a 30-year fixed mortgage at about 6.625 percent, which is what a Massachusetts bank quoted us in June 2026. Rates can change so think of this as just an example and check the current interest rates with your lender before you count on them. At that rate, the interest alone on $400,000 is about $2,200 in the first month, and your full monthly payment, principal and interest together, is about $2,561. Because early payments are mostly interest, that cost stays nearly the same every month during a build. Here's what the delay costs as time goes on.

So if it takes about 100 days to build longer than it needed to, it will cost you somewhere close to $7,400 in interest, or about $8,500 if you count full payments, on a $400,000 loan. On a larger loan or a longer delay, the number will go up from there.

There is a reason we build on a fixed schedule and put the completion dates in the contract. On these home-based loans there isn't a draw schedule to protect you like there is on a commercial construction loan where the bank releases the money in phases and the lender's timeline keeps the funds in check. When you are using an ADU refinance the calendar of the builder is the only thing that keeps you from paying the extra interest. That means you must have a builder who sticks to a tight, realistic timeline so your money is protected in a way that doesn't show up as a line on the quote. After building dozens of these, we treat a realistic schedule as a tool to control costs, and it should be in your comparison right next to the price.

Is the Cheaper Quote Really Cheaper?

Think about these two builders side by side. Suppose we give your ADU a set price and finish in about 100 days. A second builder quotes $10,000 less but tells you honestly that the job will take closer to 200 days. The lower quote wins on the estimate but it isn't always better for your bank account.

Those extra 100 days are the difference that we just put a price on. For a $400,000 loan, they will add roughly $7,400 in interest, close to $8,500 in total payments. That alone does away with most of the $10,000 you thought you were saving. If the delay goes on any further, or you have to borrow more at a higher rate, then the cheaper builder can become the more expensive one. The point isn't that a slower builder always costs more. It's that you can't know for sure which quote is actually lower until you add each builder's carrying cost to their price.

A longer build has another cost that isn't included in the quote either. Whatever your ADU is meant to do for you, house a parent, bring in rent, or let you combine two housing payments into one, it can't start doing it until it's finished. A build that drags on for an extra three or four months is three or four months that you have to wait for that benefit to start. That delay is harder to put into the quote, but it's real and like interest it will go up with time. So the honest comparison isn't between two quotes, it's quote plus carry against quote plus carry with a good idea which builder will actually keep to their timeline.

What Should You Ask a Builder Before You Compare Prices?

Because the timeline is part of the cost, it needs to be discussed early, not after you have signed. The loan officers we work with encourage the families to settle it before anything else. Taylor Stefano, a loan consultant at Loan Depot who talked with us about how to break down ADU financing, tells her clients: find out where you stand with the builder and talk to them about whether they can keep to the schedule, so you and the builder are on the same timeline. When a client isn't sure yet, her advice is to wait a couple of months and talk about those things first, instead of getting a loan with a schedule nobody has confirmed.

There are a few questions that make a price comparison a real one. Be sure to ask each builder for a specific completion window that's measured in weeks. Ask what usually controls their schedule, and how often they actually meet the dates they quote. Ask to walk through a finished ADU that the company has built so you see what kind of work their timeline produces. These answers tell you whether the schedule on the estimate is a commitment or just a maybe, and the schedule is the number that protects your carry.

The goal isn't to find the builder who promises you the earliest completion date. Anyone can promise a date. The goal is to find the builder whose schedule you actually believe. All else being equal, price will decide it. The catch is that all else isn't usually equal, and not all builders turn out to be the same.

When Is the Lower Quote the Right Call?

Since we are the builder that's telling you to look past the sticker price, you might think we'd say that the cheaper quote is always a trap. It isn't. There are times when the lower number is the right choice.

If you're paying cash, there's no loan and no interest to pay, so extra build time will cost you patience, not money. A slower builder who does excellent work might be worth the wait. If your loan is small, then the monthly interest is small too and a slight delay might not come close to the money you're saving up front. And if a lower-priced builder is really the same on quality, references, and a schedule you believe, and only slightly slower, the cost might still be in their favor once you check the numbers. Running the numbers is the whole point and sometimes they favor the cheaper builder.

We are also not the right fit for everyone. If speed's the only thing you are considering, you can find someone who'll promise a faster date than we will, because we would rather give you a schedule we can keep than one that just sounds good. What we are asking you to do is not to hire us but to compare builders on the full cost, with the carry and timeline included, so that whoever you choose, you have the real costs in front of you.

Add the Carry Before You Choose Your Builder

The builder with the lowest price isn't always the one that costs you the least. With money you borrow against your home, every extra month of construction is another month of interest, so a credible timeline is worth real dollars. Compare the full cost, price plus carry, and choose the builder whose schedule you trust. Here's how to get real numbers for your own project.