Your Neighbor Just Built an ADU. Why Can’t You?

Why Similar Lots Can Have Very Different Outcomes

Your neighbor just finished building a detached ADU with no major issues. Now you’re ready to do the same. You share the same street, same zoning, and roughly the same lot size. On paper, your project should follow the same path.

But once you get started, everything changes. Plans stall. Costs rise. New restrictions come up during design. What seemed like a straightforward process suddenly feels unpredictable.

This happens more often than you’d expect. Two lots can look identical on the surface but lead to very different outcomes. Subtle differences like slope, access, utility layout, or how local officials interpret the code can quickly complicate a project, often after time and money have already been invested.

The Assumption

Most homeowners start with the same question: “What’s my zoning?”

Cities encourage this by publishing zoning maps, hosting workshops, and promoting ADU-friendly policies. So if both you and your neighbor are zoned R1, it’s easy to assume you have the same building rights.

That’s where many projects take a wrong turn.

Zoning sets the legal framework. It tells you what type of structure is allowed and outlines basic development standards. But it doesn’t answer the more important question: Can your lot actually support what the code allows?

It’s common to confuse zoning with feasibility. In reality, zoning is just one part of a much bigger picture. Your neighbor’s ADU might have worked because of a flat lot, clear utility paths, or enough space along the side yard. If your property is missing just one of those conditions, the same project could become complicated or too costly to build at all.

Zoning tells you what’s permitted. It doesn’t tell you what it will take to make it happen.

What Makes a Lot Look the Same on Paper

At first glance, your property might seem identical to your neighbor’s. Same street. Same zoning designation R1, RD2, or another single-family category. The parcels are roughly the same size and shape. Setbacks look similar, and both lots fall under the same general plan or community overlay.

From a zoning map or city brochure, everything looks equal. That’s often what homeowners, real estate agents, and even some consultants rely on when evaluating ADU potential.

But those similarities only go so far.

Things like zoning, lot size, and frontage form the legal foundation for what might be allowed. What they don’t show are the physical and regulatory details that decide whether a project is truly possible.

Zoning maps don’t show slope, tree locations, sewer lateral paths, or fire access routes. They won’t tell you about recorded easements, utility line conflicts, or whether your side yard is 36 inches wide or 40. These may sound like small differences—but they can determine whether you have space to build at all.

Knowing your lot looks eligible is helpful. Knowing if it’s actually feasible is what really matters.

What Makes One Lot Buildable and the Other Not

Zoning may say an ADU is allowed, and your lot might look just like your neighbor’s on paper, but whether you can actually build comes down to details zoning maps don’t show.

These parcel-specific conditions often explain why one project moves forward easily while another hits delays, redesigns, or even gets denied.

Here are some of the most common factors that can make or break an ADU project:

• Slope or Grade Changes Even a small slope can add complexity. It might require a stronger foundation, drainage upgrades, or structural engineering. In hillside areas, you may also need grading plans or retaining walls, which raise both costs and permitting hurdles. In some cases, slope can reduce how much of your lot is actually usable.

• Fire Access Most cities require a clear, code-compliant path from the street to the ADU for emergency access. That usually means a 3- to 4-foot-wide walkway with no obstructions. If your side yard is too narrow or the distance too long, the fire department might ask for major changes or reject the plan entirely.

• Utility Conflicts ADUs need sewer, water, and power. If utility lines run under your planned building area or through a restricted easement, you may have to reroute them. That could mean trenching across your yard or relocating a sewer lateral—both expensive options. And if the easement blocks the build zone completely, you may have to rethink your layout.

• Overlay Zones and Local Restrictions Your lot might be in a hillside zone, high fire severity area, floodplain, or historic district even if your neighbor’s isn’t. These overlays come with extra requirements for things like height, setbacks, materials, and review procedures.

• Existing Structures and Obstacles A garage, pool, tree, or retaining wall can limit where you’re allowed to build or block the required clearances altogether. For example, a 10-foot separation between buildings might be required. If your lot doesn’t meet that, it could eliminate your preferred location entirely.

These aren’t just technicalities. They directly affect your buildable area, your design options, your timeline, and your budget. And they often don’t show up until you’re already deep into planning unless you look for them early.

How to Actually Find Out

By now, it’s clear that zoning is just one part of the equation. So how do you figure out whether your lot can realistically support an ADU before you spend money on design, surveys, or permits?

Most homeowners start with the basics: they check the city’s zoning map, skim a brochure, or talk to a contractor or Realtor. While these sources might confirm that ADUs are allowed in your zone, they rarely uncover the site-specific issues that can stall or stop a project later on.

To get an accurate picture of what’s possible, you need to dig deeper. Look for information that goes beyond zoning labels and reveals what’s happening on your specific lot:

• Parcel-level zoning and overlay zones Find out if your property is subject to additional rules due to hillside regulations, fire zones, special plans, or historic districts.

• Recorded easements These may limit where you can build, especially if utility lines or shared access routes cross the property.

• Fire access requirements Make sure there’s a clear, code-compliant path from the street to your proposed ADU location something often overlooked until plan check.

• Slope and grade changes Elevation shifts, even subtle ones, can affect your design and trigger extra engineering or drainage work.

• Environmental constraints Properties in flood zones, landslide areas, or other high-risk zones often require additional reports, permits, or design changes.

• Infrastructure and utility conflicts Knowing where your water, sewer, and electrical lines are and whether they can support a new unit can make or break your plan.

Final Thoughts: Same Zoning, Different Reality

Just because your neighbor built an ADU doesn’t mean you can. Even on the same street, with the same zoning code, two properties can have completely different outcomes.

That’s because zoning only tells part of the story. Each lot comes with its own physical constraints, overlay zones, utility layouts, and access conditions. These differences are often invisible until you’re deep into design or permitting when changes are expensive and time-consuming.

By identifying those constraints early, you can avoid setbacks and make informed decisions from the start. A clear understanding of slope, access, easements, and utility conditions can save you from delays, redesigns, and unexpected costs.

Zoning gives you permission. Your site conditions decide what’s possible.

Before you build, make sure your lot can support what you have in mind. The earlier you know, the better the outcome.