Why Your Neighbor Could Build an ADU and You Can’t

Your neighbor just finished building a detached ADU. It went smoothly, and now you’re ready to do the same. Your lot is the same size, on the same street, and in the same zoning district. It should be just as simple, right?

But once you start, the process stalls. Costs rise. Design hits roadblocks. What looked like a sure thing suddenly turns complicated.

This happens more often than you’d think. Two lots can look identical on paper and still lead to very different results. Small differences in slope, access, utility paths, or how local officials interpret the code can derail a project often after you’ve already spent time and money.

Zoning Isn’t the Whole Story

Most homeowners start with zoning. It’s the obvious first step. If you and your neighbor are both zoned R1 or RD2, it’s natural to assume you have the same building rights.

But this is where many projects hit trouble.

Zoning gives you a legal baseline. It says what you’re allowed to build and sets general rules. What it doesn’t tell you is whether your lot can actually support what the code allows.

Zoning isn’t the same as feasibility. Your neighbor’s ADU may have worked because of a wide side yard or flat grade. If your lot has even one constraint, your options could be limited or more expensive than expected.

Zoning tells you what’s allowed. It doesn’t tell you what’s possible.

Why Lots Can Look the Same, But Perform Differently

On paper, your lot and your neighbor’s may look nearly identical. Same zoning. Same block. Similar size and shape. Same setback rules. Same general plan.

To many homeowners and even some professionals that sounds like a green light.

But the similarities are only surface deep.

Things like zoning, frontage, and plan area do matter. They tell you whether an ADU can be considered. But they don’t reveal what stands in the way of actually building one.

Zoning maps won’t show you slope, tree locations, utility routes, or fire access limitations. They don’t reveal easements, physical obstructions, or whether your side yard is 36 inches or 40. These details can mean the difference between a smooth approval and a hard stop.

It’s helpful to know your lot looks eligible. It’s critical to know whether it truly is.

What Makes One Lot Buildable and the Other Not

Two lots can share zoning but face very different physical and regulatory conditions. Here are some of the most common reasons one project moves forward and the other doesn’t:

Slope or Grade Changes 

Even small elevation changes can require structural upgrades, drainage improvements, or grading work. In hillside zones, you may need retaining walls or geotechnical reports, which raise costs and extend timelines.

Fire Access 

Many cities require a clear, code-compliant path from the street to the ADU. If your side yard is too narrow or obstructed, fire access may be denied forcing a redesign or rejection.

Utility Conflicts 

ADUs need access to sewer, water, and power. If these lines run beneath your build area or through a no-build easement, rerouting them can add thousands to your budget or block the project entirely.

Overlay Zones and Local Restrictions 

Some lots fall under extra rules like hillside, high fire risk, floodplain, or historic overlays. These add restrictions on height, setbacks, access, and design. Your neighbor might be outside these zones even if they’re just a few houses away.

Existing Structures or Obstacles

Pools, garages, large trees, or retaining walls can block access or reduce buildable space. Even a small spacing rule like 10 feet between buildings can kill a layout that worked elsewhere.

How to Know What You’re Working With

If zoning isn’t enough, what is?

Most people start by checking a zoning map or asking a contractor. That’s a start, but it won’t tell you about the site conditions that really matter.

Final Thoughts: Similar Isn’t the Same

Just because your neighbor built an ADU doesn’t mean you can. Two properties with the same zoning can have very different outcomes.

That’s why early site analysis matters. Knowing your lot’s physical limits and hidden constraints before you invest in design can save you time, money, and frustration.

Zoning tells you what’s allowed. Your lot tells you what’s possible. 

Before you build, get the full picture.