What Is an ADU? And What Does It Actually Take to Build One in the Hudson Valley

By Spenser McCoy

Not every Hudson Valley property can support an accessory dwelling unit. Before you look at a single floor plan, that's the question worth answering: not what an ADU could look like, but whether your lot, your septic system, and your Town's code will actually let you build one. This piece covers both — what an ADU is, and the real sequence for getting one built.

What Is an ADU?

An accessory dwelling unit is a second, independent living space on the same lot as your primary home. It has its own kitchen, its own bathroom, and its own entrance. It is not a bonus room and it is not a finished basement with a hot plate. It's a legally distinct housing unit, and that distinction is what makes it useful: a place for an aging parent, an adult child, or a long-term tenant, without subdividing your property or moving.

Three common forms:

• Detached — a standalone structure in the backyard, often prefab or modular

• Attached — an addition connected to the main house

• Conversion — an existing garage, basement, or accessory structure reconfigured into a legal unit

Which form fits depends on your lot, not your preference. A steep grade, an undersized septic system, or a setback that eats your buildable area will rule out options before aesthetics ever enter the conversation.

What Actually Blocks an ADU

This is the part most ADU content skips, because it's less fun than a rendering. Before you spend a dollar on design, these are the questions that decide whether the project is possible at all:

• Does your Town's zoning code allow ADUs on your lot, and does it cap size, height, or setbacks in a way that changes what fits?

• Can your septic system support a second unit, or does it need to be upgraded or replaced?

• Does your lot coverage or impervious surface limit leave room to build?

• Is there an owner-occupancy requirement, a local moratorium, or a site plan review step that adds time?

Every one of these is Town-specific and property-specific. Generic ADU content that quotes a single size limit or a single setback figure for "the Hudson Valley" is wrong on purpose or wrong by accident, because the answer changes from one municipality to the next, sometimes from one parcel to the next. This is the reason a feasibility study exists before design work starts: to confirm the actual constraints on your specific property, against the actual code that governs it, before you're emotionally or financially committed to a floor plan that may not survive contact with your Town.

How to Build an ADU: The Real Sequence

1. Confirm what your property and your Town allow. Before any design spend, get a written answer on zoning eligibility, septic capacity, setbacks, and coverage for your specific lot. This is what OneBuild's Feasibility Study does: a flat-fee engagement that tells you what your property can support, credited toward the build if you move forward, and worth exactly as much if the honest answer is that you shouldn't build there. That's happened. We've closed a site out as unbuildable on three independent blockers and advised the owner toward selling instead of building.

2. Design to the site and the code, not the other way around. Once you know what's buildable, design decisions — unit size, layout, manufacturer or model selection — get made against real constraints instead of guesswork. This is where a design background matters.

3. Run the permitting and agency process. Site plan review, building permits, and in some cases DEC or Army Corps review for wetlands, depending on the site. This step is where projects stall when nobody owns it. OneBuild coordinates from the first question through the Certificate of Occupancy.

4. Select and contract directly with your builder and manufacturer. OneBuild is not a general contractor. We coordinate the process; you contract and pay directly with the manufacturer or the builder. We take no commission from manufacturers or contractors on any project, which means the recommendation you get is the one that fits your site and your budget, not the one that pays us.

5. Build and inspect through Certificate of Occupancy. Construction, inspections, and closeout, coordinated against the plan and the permit set that was built for your property in step 1, not a generic template.

Where OneBuild Fits

OneBuild's fees are flat, defined by scope, and agreed in writing before work starts, staged in two parts: a Coordination Fee and a Design Fee. We don't mark up construction or pass-through costs, and we take nothing from manufacturers or contractors, so the advice can follow the facts on your property, not our incentives.

If you're not sure whether your property can support an ADU, that's the first question to answer, before you spend on anything else.

Book a free Discovery Call. The next step from there is the Feasibility Study, credited toward your build.

One Team. One Process. OneBuild.