The short version
Affordability isn't one number, it's two. There's what a lender will approve you for, and there's what you can actually live with. Those are rarely the same, and the gap is wider in Sonoma County than most places because of two line items people forget: property taxes and home insurance. I've watched a buyer get pre-approved for $780,000 and then find out the insurance quote on the exact house they wanted added $400 a month to the payment. Suddenly the affordable house is a $700,000 house.
So the real answer is: start with your income and debts to find the lender's ceiling, then subtract for taxes and insurance out here specifically, and only then do you know your number. A calculator that just asks price, down payment, and rate is lying to you by omission. Below is how the actual math works, and how to get to a number you can trust before you fall in love with a listing you can't carry.
How does a lender decide what I can afford?
It comes down to one thing more than any other: debt-to-income ratio, or DTI. The lender takes your monthly debt payments — the new house payment plus car loans, credit card minimums, student loans, anything that shows on your credit report — and divides it by your gross monthly income, before taxes. That percentage is the whole game. Not your bank balance, not what you feel comfortable with. The ratio.
Most loan programs want your total DTI somewhere around 43% to 50%, depending on the program and the strength of the rest of your file. FHA tends to stretch higher, conventional a bit tighter, and a strong credit score and reserves can push the ceiling up. What counts as income is broader than a lot of people assume — base pay, sure, but also consistent overtime, bonus, commission with a two-year history, and self-employment income after write-offs. What doesn't count is the money you think you'll make next year. Lenders work off what you can document, not what you project.
- DTI is your total monthly debt payments divided by gross (pre-tax) monthly income.
- The new mortgage payment includes principal, interest, property taxes, and insurance — not just the loan.
- Most programs land around a 43%–50% DTI ceiling; verify what your specific program allows.
- Documentable income counts; a raise or a side gig without history usually doesn't yet.
What's the 28/36 rule, and does it still hold?
The old guideline says spend no more than 28% of your gross income on the house payment, and no more than 36% on all your debt combined. It's a useful gut check, and honestly it's closer to what you can comfortably live with than what a lender will actually let you borrow. Lenders will approve you well past 36% total these days. Whether you should go there is a different question.
Here's how I frame it for clients. The lender's max is the edge of the cliff. The 28/36 zone is where you still have room to breathe — to save, to handle a surprise, to not resent your house. In a high-cost area like ours, a lot of buyers do end up stretching past 28% on the housing piece because that's just what it costs to get in. That can be fine. But do it with your eyes open, and leave yourself a cushion, because the next two sections are the expenses that eat the cushion alive.
What does Sonoma County add to the monthly payment?
Every mortgage payment out here carries property taxes baked in, and California's system is specific. Under Proposition 13, your base rate is roughly 1% of the purchase price, but the actual bill lands higher — usually around 1.1% to 1.25% once you add voter-approved local bonds and assessments. On an $800,000 home that's north of $8,000 a year, close to $700 a month, and it's escrowed into your payment whether you think about it or not.
Then watch for two extras. Some newer subdivisions — parts of Windsor, Rohnert Park, the newer Santa Rosa developments — carry Mello-Roos assessments that fund infrastructure and can add real money on top of the base tax. And if you're buying a condo, a townhome, or anything in a planned community, there's an HOA due every month that the lender counts against your DTI just like a car payment. A $350 HOA doesn't just cost $350 — it lowers how much house you qualify for. I always pull the actual tax bill and HOA figure on a specific property before we call anything affordable, because the estimates buyers carry in their heads are almost always low.
- Property tax runs roughly 1.1%–1.25% of purchase price a year here (base 1% plus local add-ons) — verify per property.
- Newer developments may carry Mello-Roos assessments on top of the base tax.
- HOA dues count against your DTI and shrink your qualifying amount dollar for dollar.
- Always price the real tax bill and HOA on the actual home, not a rule of thumb.
Why is home insurance the affordability killer out here?
This is the one that's changed the math in Sonoma County, and it's the one I make sure every buyer understands before they shop. After the 2017 fires and everything since, homeowners insurance in wildfire-exposed areas has gotten expensive and, in some pockets, hard to get at all. A policy that might run $1,500 a year in a low-risk part of the country can run several thousand here, and if the home sits in a high fire-risk zone, you may end up on the California FAIR Plan plus a separate wrap-around policy — two premiums instead of one.
Why it matters for affordability: your lender requires insurance, it's escrowed into your monthly payment, and it counts in the qualifying math. So a house in a wildfire zone doesn't just cost more to insure — it literally lowers how much you can borrow, because more of your DTI budget gets eaten by the premium. I've had two nearly identical homes, similar price, where the one up a wooded road cost $300 to $400 more a month to insure than the one on the valley floor. That's a real difference in what you can afford. My rule: get an actual insurance quote early, before you're emotionally committed. Not an estimate — a real quote on the real address. It can make or break the deal, and it's better to know in week one than at closing.
- Insurance is required, escrowed, and counted in your DTI — a high premium lowers your buying power.
- High fire-risk properties can require the FAIR Plan plus a separate policy, doubling the paperwork and cost.
- Two similar homes can have very different premiums based purely on fire-zone rating.
- Get a real insurance quote on the specific address early — treat it like a contingency, not an afterthought.
It's not just the payment — what cash do I need up front?
Affording the monthly payment is half of it. The other half is having the cash to get to the closing table. Down payment is the obvious piece, and it's more flexible than people think — 3% to 3.5% on conventional and FHA, zero down on VA and USDA if you qualify. But the down payment isn't the only cash you need.
Closing costs typically run another 2% to 5% of the purchase price — lender fees, title, escrow, prepaid taxes and insurance, the appraisal. Some of that can be covered by a seller credit if we negotiate it into the offer, which is a lever a lot of buyers don't know they can pull. And most programs want to see reserves: a few months of mortgage payments sitting in the bank after closing, proving you won't be wiped out the moment something breaks. The good news is the down payment and a chunk of closing costs can usually come from a documented gift from family. The point is to map the total cash-to-close before you shop, not discover it three weeks before escrow closes.
How do I find my actual number before I start looking?
Get pre-approved, but get the right kind of pre-approval — one that accounts for the Sonoma County reality, not a generic online estimate. When I pre-approve a buyer, we don't just find the lender's ceiling. We back into a monthly payment you're actually comfortable with, then work out what price that supports once we've layered in realistic property taxes and an honest insurance estimate for the areas you're considering. That's the number you shop with. It's usually a little lower than the maximum, and buyers are almost always relieved to have it, because it means every house they tour is one they can actually carry.
The reason to do this with a broker instead of one bank: I can shop your file across multiple lenders, and the program you land on changes your DTI ceiling and your payment. A buyer who's tight on ratios might qualify comfortably on one program and get declined on another for the same house. That flexibility is exactly what turns a 'maybe' into a 'yes' in a competitive market. Let's build your real number first — then go find the house.