How to Finance an ADU: Every Option Compared

How should you finance an ADU? Two facts decide it before any product does: do you have the equity today, and are you protecting a cheap first mortgage?

• Have the equity + a sub-4% first mortgage? A HELOC (7.47% avg) or home equity loan (8.13%) is usually cheapest, because it leaves that cheap mortgage alone.
• Don’t have the equity? Only a loan that underwrites the finished value reaches the build: FHA 203(k), Fannie HomeStyle, or a construction-to-permanent loan (high-6% to ~9%). Most reprice your first mortgage.
• First mortgage already at today’s rate? A cash-out refinance (~7%) becomes competitive, because there’s no cheap rate left to protect.
• Small, fixed scope, want no lien? A personal loan (~12%+) can fit, but rarely on a six-figure build.

TL;DR: No single product is the best way to finance an ADU; the right one is set by your equity and your existing mortgage rate, not the ADU. If you have the equity, a HELOC (7.47% average, per Bankrate) or home equity loan (8.13%) is usually cheapest because it borrows only against your equity and leaves your first mortgage untouched. If you don’t, a renovation or construction loan (FHA 203(k), Fannie Mae HomeStyle, or construction-to-permanent at high-6% to ~9%) underwrites the finished value but typically reprices your first mortgage; a cash-out refinance (~7%) only makes sense if that rate is already at today’s level. The cost most comparisons skip is that repricing, about $15,000 a year on a $400,000 balance moving from 3.25% to 7%.

Most ADU-financing guides line up seven products and describe each one. That is the part that’s already commoditized; the products are not a secret. What’s missing is the decision logic that connects your situation to the right one. The honest truth is that the ADU barely matters to the choice. Two facts about you decide almost everything: whether you have the equity today, and what rate your current first mortgage carries. This page compares every realistic option with current rates and honest tradeoffs, then maps borrower situations to products so you can find your row instead of reading seven brochures. Building an ADU is among the highest-value ways to use a HELOC, and for the deep dual-math on the two most common paths, the HELOC versus construction loan piece runs it dollar for dollar. For the product basics first, start with how a HELOC works, and for the broader equity menu see the comparison hub.

The seven ways to finance an ADU, compared

Every product below funds an ADU. They differ on three things that decide the money: how the rate is set, whether the loan is capped by your current equity or the finished value, and whether it touches your existing first mortgage. That third column is the one most comparisons bury, and it’s usually the most expensive line in the whole decision.

Product	How it prices	Rate (June 2026)	Best when	The catch
HELOC	Variable: prime + margin	7.47% avg (Bankrate)	You have the equity, want to draw as you build, and are protecting a low first mortgage	Variable rate; payment jumps when the interest-only draw period ends; capped at current equity
Home equity loan	Fixed, lump sum	8.13% (Bankrate)	You want a fixed payment and know the full build cost up front	Whole sum accrues interest from day one; capped at current equity; no draw flexibility
Cash-out refinance	Fixed, replaces first mortgage	~7% (near PMMS 6.52%)	Your current first-mortgage rate is already at or above today’s	Reprices your entire first mortgage at today’s rate — ruinous if yours is cheap
Construction-to-permanent	Fixed after conversion; sized on finished value	high-6% to ~9%	You lack current equity, want one fixed payment, building a large/uncertain project	Usually wraps your first mortgage; inspections and draw oversight; single-close locks today’s rate on the whole balance
FHA 203(k) (Standard)	Fixed, sized on after-improvement value	tracks FHA 30-yr	Low equity, lower credit (580+), can use projected rent to qualify	Replaces your first mortgage; mortgage insurance; Standard 203(k) requires a HUD consultant
Fannie Mae HomeStyle	Fixed, sized on as-completed value	tracks conventional 30-yr	Low equity, conventional borrower, detached ADU allowed	Replaces your first mortgage; 5% down typical; renovation-loan process friction
After-renovation-value second lien (RenoFi-style)	Fixed, sized on finished value, sits behind first mortgage	varies; typically above HELOC rates	Low equity and protecting a low first mortgage	Newer, fewer lenders; second-lien pricing; underwriting on projected value
Personal loan	Fixed, unsecured	~12% avg, 8–36% range (Bankrate)	Small fixed scope; you want speed and no lien on the home	High rate, short term, heavy payment; rarely works on a six-figure build

Two patterns run through that table. The products capped by your current equity (HELOC, home equity loan) are the cheap-but-limited group: lowest effective cost, but they stop at the equity you have today. The products sized on the finished value (203(k), HomeStyle, construction-to-permanent, after-renovation-value second liens) are the reach group: they can fund a build your equity can’t, but most of them refinance your first mortgage to do it. The personal loan sits outside both, trading collateral for speed at a much higher rate. Where you land depends on which constraint is actually binding for you, and the next section sorts that out.

Which product fits your situation? The decision framework

Skip the menu and find your row. Four situations cover nearly every ADU borrower, and each points to a different product for a reason that traces to mechanics, not preference.

You have the equity, and a low first mortgage. This is the most common California case, and the HELOC is usually the answer. Not because its rate beats the others (it often doesn’t) but because it borrows only against your equity layer and leaves your sub-4% first mortgage completely alone. Drawing as the build bills, paying interest only on what you’ve drawn, is a bonus that happens to match how construction actually invoices. A fixed-rate home equity loan is the close substitute if you want payment certainty over draw flexibility and you already know the full cost. The decision rule: when you’re protecting cheap existing debt, the product that doesn’t touch it almost always wins, even at a slightly higher rate on the new money. The full case is in can you use a HELOC for an ADU. Test whether your own build pencils — equity against the build cost, rent against the repayment payment — in the ADU Feasibility Analyzer.

You have the equity, but your first mortgage is already at today’s rate. Bought or refinanced recently at 6.5%? Then there’s no cheap rate to protect, and the HELOC’s core advantage evaporates. Now a cash-out refinance becomes genuinely competitive, because rolling everything into one fixed loan costs you nothing in repricing and may carry a lower rate than a HELOC’s variable pricing. This is the one scenario where replacing your first mortgage is not a penalty. Run it in the HELOC vs. cash-out calculator before assuming the second-lien path is cheaper.

You don’t have enough current equity. A HELOC and a home equity loan are both off the table here, full stop, because they’re capped at today’s value and can’t see the unit you’re about to build. The reach products are your only path: FHA Standard 203(k), Fannie Mae HomeStyle, or a construction-to-permanent loan, all of which underwrite the after-completion value. FHA also lets you count 50% of the new ADU’s projected rent toward qualifying income, per HUD Mortgagee Letter 2023-17, which can be what gets a low-equity owner approved. The catch is that most of these refinance your first mortgage. If you’re also protecting a low rate, ask specifically about an after-renovation-value second lien, the one structure that reaches the build without repricing your cheap money. The whole low-equity path is mapped in financing an ADU with no equity.

The scope is small and fixed. For a sub-$150,000 garage or junior-ADU conversion, the calculus shifts. A six-figure revolving HELOC may be more product than a small, well-defined build needs. A fixed home equity loan, or even a personal loan if you want to keep the house lien-free and fund fast, can be the cleaner fit. The cheapest build doesn’t always call for the most flexible financing. The conversion-specific math lives in garage conversion ADU financing.

A worked example: HELOC vs. construction-to-permanent on a $300,000 build

Take the most common matchup, a HELOC against a construction-to-permanent loan, on a realistic California build. The point here is the shape of the difference; the line-by-line dual-math lives in the dedicated comparison.

The setup: a $900,000 home with a $400,000 first mortgage locked at 3.25% a few years ago, and an owner who wants to build a $300,000 detached ADU. They have enough equity for either path, so this is purely a cost question.

	HELOC path (keep first mortgage)	Construction-to-permanent (refinance everything)
First mortgage	$400,000 stays at 3.25%	Paid off and rolled into the new loan
New financing	$300,000 HELOC at 7.47% (variable)	$700,000 at ~7.0% fixed
First-year interest, first mortgage	~$13,000	— (now inside the $700k)
First-year interest, ADU money	~$22,290 (interest-only draw)	— (inside the $700k)
First-year interest, total	~$35,290	~$49,000
What moved	Nothing on the old loan	The $400k repriced from 3.25% to 7%

The gap is about $13,700 in the first year, and nearly all of it traces to a single line: the $400,000 that got repriced. Borrowing $300,000 against the home at roughly 7% to build costs about the same either way. What differs is whether you also reprice the money you’re already borrowing cheaply. Repricing that $400,000 from 3.25% to 7% costs $15,000 a year on its own ($400,000 × 3.75%), and you pay it every year the new loan is outstanding, not just the first.

So the decision hinge isn’t the ADU; it’s the first mortgage. Protecting a sub-4% first mortgage is usually where the HELOC pulls ahead, by more than the rate sheets suggest. Flip the owner’s first-mortgage rate to today’s level, say they bought recently at 6.5%, and the repricing penalty mostly disappears, at which point the construction-to-permanent loan’s single fixed payment and higher borrowing capacity often win outright. The hinge swings on your number. Model your own version in the HELOC payment calculator before you assume the single payment is the cheaper one.

The catch nobody puts in the brochure: the payment phases

One more number decides whether a HELOC build is sustainable, and it’s not the rate. It’s the jump between the two payment phases, the one lender illustrations lead with the cheap half of.

During the draw period, a HELOC is interest-only: that same $300,000 balance at 7.47% runs about $1,858 a month. When the draw period ends, the balance amortizes over the repayment term, and at a 20-year payback the payment jumps to roughly $2,404 a month, a 29% increase. Budget around the repayment number, never the interest-only teaser, especially if you’re counting on the ADU’s rent to help cover it. And because the rate floats with prime (6.75% as of June 2026, per JPMorganChase), every quarter-point move shifts the payment on $300,000 by about $62 a month in either direction. That’s not a footnote on a balance this size; stress-test the payment a point or two higher before you commit to a build budget.

The fixed-rate products (home equity loan, the permanent leg of a construction loan, a cash-out refinance) trade that flexibility away for a payment that doesn’t move. Which tradeoff is right isn’t a matter of taste; it’s a matter of whether your household budget can absorb a variable payment that resets every time the Fed acts. If it can’t, the certainty of a fixed product has real, nameable value. And if you build with a HELOC and later want to retire it before that jump, refinancing after the ADU is built walks the keep-your-first-mortgage math.

Does California change the answer?

It sharpens it. California is where this question gets asked most, for three structural reasons.

The state requires cities to approve qualifying ADUs ministerially, per California HCD, so the build is more feasible here than almost anywhere else. California’s high home values mean equity-rich owners are common, which tilts more Californians toward the HELOC or home equity loan path than you’d see nationally. And Prop 13 changes the math underneath every option: borrowing against your home, by any product on this page, is not a change of ownership and never resets your assessed value. Building the ADU adds the new unit’s value to your assessment (a partial reassessment of just the new construction), while the rest of your home keeps its protected basis. The financing choice doesn’t affect that; only the building does. The full interaction is in the California HELOC guide.

The honest bottom line is the same one that governs every smart use of home equity: the cheapest path is rarely the lowest rate on the new money. It’s the one that doesn’t quietly reprice the cheap money you already have, and it’s the one whose payment your budget can actually carry once the teaser phase ends. Sort your two facts first, current equity and current first-mortgage rate, and the seven-product menu collapses to one or two real candidates before a lender ever frames the decision around project size.

Rates cited are national averages or representative ranges as of June 2026 and change frequently; construction and renovation loan pricing varies widely by lender, program, and structure, and your offered rate depends on your credit, equity, and program. This article is general information, not legal, tax, or financial advice, and not a recommendation of any specific transaction. Confirm program terms and tax treatment with the relevant agency and a California-licensed professional about your situation.