Adding a unit to a property you already own, or to one you are buying, is one of the few genuinely durable value-add plays left in expensive markets. It is also a project that confuses financing, because it sits between a renovation and a ground-up build and it is valued differently from both. Here is how the money works.
What you are actually building
An accessory dwelling unit is a self contained second residence on a lot that already has, or will have, a primary home. In practice it takes three forms: a conversion of existing space such as a garage or basement, an attached addition, or a detached structure built new on the lot.
Those three are different construction projects with different costs, different permitting paths and different timelines, and lenders treat them accordingly. A garage conversion is closer to a heavy renovation. A detached new build is closer to ground-up construction, with its own foundation, utilities and inspections.
Permits decide almost everything
This is the point worth reading twice. A permitted unit is a unit. An unpermitted one is usually just extra square footage in the eyes of an appraiser, and sometimes a liability.
Rules for what can be built, at what size, with what setbacks and parking, vary by state, by city and sometimes by parcel, and they change. Confirm what your jurisdiction allows for your specific lot with the local planning department before you underwrite anything. What is consistent everywhere is the financing consequence: a lender funds against value, value follows the appraisal, and the appraisal follows the permit.
Buying a property with an existing unpermitted conversion is a related situation. Ask whether it can be legalized, at what cost and on what timeline, and price the property as though the answer is no until you have evidence.
How the property will be valued
A one to four unit residential property is generally valued using the sales comparison approach, from closed sales of similar properties in the area. That is different from a five unit or larger property, which is typically valued on its income using a capitalization rate.
The practical consequence is important. On a one to four unit property with an ADU, the value comes from what comparable properties with similar accommodation have actually sold for, not from a calculation on the rent. If there are few comparable sales of homes with permitted ADUs in your area, the appraisal will be conservative, and your loan will follow it.
Two disciplines follow. Check the comparable set before you commit to the scope. And do not over-improve, because an ADU finished well above what the neighborhood pays for is budget converted into nothing. Our post on how a lender values your property covers what the appraisal actually examines.
Financing the build
A bank will rarely fund an ADU build cleanly, because the property changes during the project and standard renovation products are not designed for new construction on an occupied lot. Private money handles it in one of three ways.
A ground-up construction loan. For a detached new unit, our construction program publishes LTC up to 80% including the lot, the build and the interest reserve, terms of 12 to 18 months, a minimum FICO of 600 and two prior ground-up projects. The build money is released in draws against completed work.
A fix and flip loan. For conversions and attached additions where the work reads as renovation. Our fix and flip program publishes up to 85% of the purchase, 100% of the rehab and 75% of ARV, terms of 6 to 18 months and rates from 9.99% to 12.00%, and it allows first time flippers.
A bridge loan or a second mortgage against equity you already hold. If the ADU is going on a property you own outright or with substantial equity, borrowing against that equity funds the build without disturbing an existing first mortgage. Our bridge program publishes LTV up to 80%, with up to 75% LTV on cash-out, and a second mortgage sits behind a first you want to keep.
Which one fits depends on the scope, the existing debt and whether you are buying the property or already own it.
Budget the way a lender does
Three lines that ADU budgets routinely understate.
Utilities and site work. A detached unit may need its own connections, a panel upgrade, trenching and drainage. On a conversion, the plumbing route is often the expensive surprise.
Soft costs. Design, engineering, permit fees, plan check and inspections all land before a single wall goes up, and they are real money in the first months.
Contingency. On a project attached to an existing structure, the chance of discovering something behind a wall is high. Our post on building a construction budget a lender funds sets out how to assemble a budget that survives underwriting.
Plan the exit before you break ground
The construction loan is short. What repays it is either a sale or a refinance, and on an ADU project it is usually a refinance.
Check the takeout before you start. Our rental DSCR program publishes 5, 7, 30 and 40 year fixed terms, LTV up to 80%, a 620 minimum credit score and first time investors allowed. Take the projected finished value, apply that leverage cap and see whether the resulting loan clears the construction balance. Then check the income against the coverage requirement using the rent the finished property can actually document.
If the numbers do not clear with room, the answer is to change the scope or the leverage now. Our post on exit strategy covers testing both routes properly.
Where to start
The construction program page carries the published terms and the property types hub shows the rest of what we lend on. If you have a lot, a plan and a question about whether the project finances, call (619) 617-2797 or open a file on the borrow page.
