The Real Monthly Payment on a $300k ADU: Math for NH Homeowners

The real monthly payment on a $300,000 ADU is not one number — it's three inputs run through a calculator: the amount you actually finance, your lender's rate on the day you lock, and the term you choose. Anyone quoting you a single payment without those three inputs is guessing, but with them you can compute your own answer in about five minutes, and this article shows you exactly how.

That matters because the monthly payment is the question that stops most New Hampshire families before they ever call a builder. The total price tag feels abstract; the monthly number is what has to fit next to the mortgage, the groceries, and the car payment. So let's take the fear apart piece by piece.

Why is $300,000 the right number to plan around?

Typical New Hampshire ADU projects run $275,000 to $400,000+ depending on design — size, finishes, site conditions, and whether the unit is attached or detached all move the total. A $300,000 build sits in the realistic middle of that range, which makes it a useful worked example: if your project comes in leaner, every number below shrinks; if you're building at the top of the range, they grow proportionally.

One thing worth knowing before you budget: since July 1, 2025, state law (RSA 674:72, amended by House Bill 577, 2025 Chapter 197) allows one ADU by right — attached or detached — wherever single-family homes are permitted. No special exception, no variance, no conditional-use permit for the ADU itself. You still need a normal building permit and still meet reasonable local standards like setbacks and septic capacity, but you are not budgeting for a discretionary-approval fight. The state law guide covers what changed in detail.

What actually determines the monthly payment?

Three levers, in order of how much control you have over them:

• Principal — the amount you finance, not the project cost. If you put $50,000 of savings into a $300,000 build, you're financing $250,000, and every downstream number drops with it.
• Rate — set by the market and your lender on the day you lock, and different for each loan product. This is the input you cannot get from an article; you get it from a phone call.
• Term — how many years you spread repayment over. Longer terms mean a lower monthly payment and more total interest; shorter terms mean the reverse.

There's a fourth, structural factor: how the loan is built. An amortizing loan (fixed payment, principal plus interest) behaves very differently month to month than a line of credit with an interest-only draw period. More on that below.

How do I run the math with my own lender's rate?

You do not need special software — any free online amortization calculator works. The process:

1. Set your principal. Start from your realistic project budget and subtract any cash you plan to contribute.
2. Call a lender and ask for today's rate on the specific product you're considering — a home-equity loan, HELOC, cash-out refinance, and construction loan will each carry different rates, and some are variable rather than fixed.
3. Pick a term — commonly somewhere between 10 and 30 years depending on the product.
4. Enter principal, rate, and term into the calculator. The payment it returns is your answer.
5. Run it three ways: your base case, a shorter term, and a smaller principal. Seeing how the payment moves teaches you more than any single quote.

Two follow-up questions worth asking the lender: what's the total interest paid over the life of each option, and is the rate fixed for the full term or can it adjust? A payment that starts comfortable and floats upward is a different commitment than one that's locked.

Which loan type fits which situation?

Most New Hampshire homeowners fund an ADU with home equity in one of four structures:

• Home-equity loan — a fixed lump sum, typically at a fixed rate, repaid in equal monthly payments. The most predictable option: one payment, known on day one.
• HELOC (home-equity line of credit) — you draw funds as construction bills come due, commonly paying interest only on what you've drawn during the draw period. Early payments look small; the payment steps up when repayment of principal begins. Rates are often variable.
• Cash-out refinance — replaces your existing mortgage with a larger one and hands you the difference. Whether this helps or hurts depends heavily on the rate you currently hold versus the rate you'd refinance into; if your existing mortgage rate is lower than today's, you'd be repricing your entire home loan to fund the ADU.
• Construction loan — designed for building: interest-only on drawn funds during construction, then converted or refinanced into a permanent loan when the unit is done.

Some lenders will also count projected ADU rental income when qualifying you for the loan — which can meaningfully change what you're approved for. Not all do, and the rules vary, so ask directly. A local lender who knows ADUs is worth the extra phone call.

How does rent change what the payment actually costs?

Here's the reframe that changes the math: the payment is not the cost. The cost is the payment minus what the unit earns or saves you.

If you rent the ADU long-term, your real monthly carrying cost is roughly: loan payment, plus insurance, plus maintenance and any property-tax change, minus rent collected. New Hampshire's housing shortage is well documented, and small, well-located units are exactly what the rental market lacks — but don't budget from a statewide average. Look up what comparable one-bedroom units actually list for in your own town this month, then haircut it for vacancy and upkeep. If realistic local rent covers most of the payment, the ADU is closer to a self-funding asset than a second mortgage.

Be honest in the model, though: budget for some vacant months, for repairs, and for the possibility that your assessment (and property tax bill) rises with the new square footage. A plan that only works at full occupancy and zero maintenance isn't a plan.

What if the ADU is for a parent, not a tenant?

Then the comparison isn't payment versus rent — it's payment versus the alternative. Assisted living in New Hampshire runs $70,000 to $100,000+ per year in commonly cited industry surveys, paid out monthly and building equity in nothing. An ADU loan payment, by contrast, is buying a permanent asset on your own property — one that can house a parent today, earn rent later, and add value whenever you sell. Under state law, towns can't require the occupant to be a family member, so the unit's use can change as your family's needs do.

Where does your town come into the math?

The financing is statewide; the buildable envelope is local. Any size cap your town sets (never below 750 sq ft under state law), setbacks, and septic requirements shape what you can build — which shapes the budget, which shapes the payment. Start with our town guides, check your county page — like Rockingham or Hillsborough — and read your town's breakdown before you set a budget, because many towns' posted ordinances still contain language the 2025 state law overrides.

Run your own three numbers through a calculator this week — the payment you compute yourself is almost never as scary as the one you imagined.