Where the money for an ADU should come from, given the appraiser might not agree with the cost

The core tension in funding an ADU is that the money going in and the value coming out are measured by two different people who never talk to each other. There are four common ways owners fund a build, say $180k for a backyard unit. Cash: no underwriting, no draws, no interest, but $180k of dry powder tied up in an illiquid improvement on one parcel, and any cost overrun has to come from somewhere else too. A home equity line on the main house: fast, flexible draws, carrying only what has been pulled. The rate floats, and lines can be reduced or frozen by the lender under conditions written into the agreement, which is worth reading closely and confirming in writing before relying on it as construction money. A renovation or construction loan underwriting to completed value: this is the option where projected finished value does real work up front, though it comes with draw inspections, contingency requirements, and a builder who has to be acceptable to the lender. A cash-out refinance before starting: prices the entire existing balance at today's rate to access the equity. The thread running through all four options is whether the completed unit appraises anywhere near what it cost. In markets where detached ADUs trade often enough to generate paired sales, appraisers have real comps to work from. In markets where a given unit is one of only a few in the county, the cost approach and appraiser judgment carry more weight, and $180k of cost does not reliably become $180k of appraised value. Some states do not publish sale prices at all, which makes the comp problem harder still. There is also a mortgage-qualification path where projected ADU income can count toward qualifying on an owner-occupied purchase, subject to caps and effective dates that need confirming with the lender in writing, though that only applies to a purchase, not a build on a house already owned. Given all that, the choice of funding source should weigh appraisal risk directly rather than treating it as a formality, since it determines how much of the build cost actually shows up as equity afterward.