≈ $22,500 down
Ongoing housing costs
PMI is included below since the down payment is under 20%.
Estimated total
$1,019/mo
$966 payment + $53 PMI
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By Alex Diaz · How we calculate this
Building a guest house, or accessory dwelling unit, is usually financed through a construction loan, home equity loan, or cash-out refinance rather than a standard purchase mortgage, and not every property is even zoned to allow one. This calculator turns the guest house's price, down payment, interest rate, and loan term into a monthly payment, then weighs that against your income to gauge how risky the purchase would be.
These key factors affect guest house affordability
Income and expenses
A guest house payment lands on top of whatever you're already paying, your primary mortgage most of all, so we weigh it against your full income and debt picture rather than the isolated cost of the build. Construction financing also tends to disburse in stages, which means the payment you're comparing here is really an estimate of what the loan looks like once it's fully drawn and converted to permanent financing. Adding a structure like this can trigger a property tax reassessment in some areas too, so it's worth checking with your local assessor rather than being surprised by a bigger bill after it's built.
Credit score
Because a guest house is rarely financed with a standard purchase mortgage, most owners lean on a construction loan, a home equity loan, or a cash-out refinance instead, and your credit score moves the rate on every one of those paths. A construction loan in particular tends to draw tighter underwriting than a plain mortgage, since the lender is financing something that doesn't exist yet.
Permits and zoning
Zoning is the first thing to check, not the last. Some cities cap the size of a secondary structure, require it to share utilities with the main house, or won't permit a full kitchen in what they'll only allow as a guest house rather than a legal accessory dwelling unit. The rules have loosened in a number of states recently, California among them, where statewide law now overrides many local restrictions and requires most cities to allow an ADU by right on a residential lot. Even so, requirements still vary enough by city that it's worth a call to the local planning department before you price out the build.
Rental income potential
Call it a guest house, an in-law suite, a granny flat, or a casita, the appeal is often the same: rent it out and offset some of what it cost to build. Model that income conservatively here, though. A vacancy month or two, cleaning and platform fees if you go short-term, and a slow season here and there are all normal, and assuming a fully booked calendar from day one is the easiest way to end up short.
How to use your results
Try adjusting the inputs to see what actually moves your risk score:
- Down payment and loan term - see how each moves your monthly payment and risk score.
- Rental income - model a conservative estimate if you plan to rent it out.
- Existing debt - add what you're already carrying for a more realistic picture.
Ways to increase how much guest house you can afford
- Pay down existing debt before taking on construction or home equity financing.
- Confirm zoning and permit requirements before pricing the project, to avoid surprises later.
- Save for a larger down payment to shrink the loan and monthly payment.
- Get quotes from a few builders who specialize in accessory dwelling units.
More housing calculators
Peer comparison data
Bureau of Labor Statistics (median income by age), Experian (credit score by age)
Used by the “how you compare” figures in the sidebar, not by the costs on this page.
Every figure on this site, with its source and the date it was last verified
How you compare
Income
Credit score
Nothing added yet - add something to see your risk.