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

Your neighbor built a detached ADU without much trouble. Now you’re ready to follow their lead. Same street. Same zoning. Same lot size.

But as soon as you try to move forward, things get complicated. Design hits a wall sometimes literally. Plans stall. Costs rise. The process that looked simple suddenly isn’t.

If this sounds familiar, you’re not the only one. This post breaks down why two seemingly identical lots can lead to very different outcomes and how to catch the differences early, before they derail your project.

The Assumption

The first question most homeowners ask is, “What’s my zoning?”

Cities make this easy. They publish zoning maps and distribute flyers that promote ADUs as straightforward additions. So if you and your neighbor are both zoned R1, it’s natural to assume your options are the same.

But zoning is just the starting point.

It tells you whether building is allowed not whether it’s actually doable. Once you get into the details, what looks simple on paper can turn out to be far more complicated.

What Makes a Lot Look the Same on Paper

Two lots can look nearly identical at first glance:

• Same zoning (R1, RD2, etc.)

• Similar square footage

• Same street frontage and lot width

• Matching setbacks

• Same neighborhood and general plan area

These details are real and relevant. But they only tell part of the story.

What they don’t show are the physical and regulatory conditions that actually determine whether an ADU can be built. And those conditions often aren’t visible on any public map.

What Makes One Lot Buildable and the Other Not

Two lots may look the same on paper, but hidden conditions can lead to very different outcomes. Here are a few of the most common issues that can make or break an ADU project:

• Slope or grade changes Even a slight slope can affect your foundation, drainage needs, or how height limits are measured.

• Fire access Most cities require a clear path from the street to the ADU. If your side yard is too narrow or the unit is set too far back, your plan may not meet code.

• Utility conflicts Sewer, water, or electrical lines might run through your build area. Trenching or rerouting can be costly or impossible if there’s a utility easement.

• Overlay zones Hillside areas, flood zones, and historic districts often come with added restrictions that impact design, materials, or placement.

• Existing structures Pools, garages, trees, or retaining walls can block buildable space or required clearances.

These issues aren’t always obvious at first. But they can affect your timeline, budget, and whether the ADU can be built at all.

How to Actually Find Out

Most homeowners begin by looking at zoning summaries or basic property reports. These can confirm whether ADUs are allowed—but they don’t reveal the physical or regulatory conditions that determine if building is actually possible.

To understand what your lot can support, you need to look deeper. Key factors include:

• Parcel-level zoning and overlay zones

• Recorded easements that restrict buildable areas

• Fire access and emergency clearance paths

• Slope and grade that affect foundations and layout

• Environmental hazards like flood zones or landslides

• Utility infrastructure and potential conflicts with sewer or service lines

Without this information, you’re planning in the dark. Before hiring an architect, pulling permits, or committing to a layout, it’s important to get a full picture of your site conditions. Doing that early can help you avoid costly revisions and wasted time.

Final Thoughts: Don’t Assume Your Lot Can Do the Same

Your neighbor’s successful ADU project doesn’t guarantee yours will be just as straightforward. Even next-door lots can have very different limitations most of which don’t show up until you’re deep into design or permitting.

The sooner you uncover those constraints, the better. It can save you time, money, and unnecessary revisions.

Zoning gives you permission.

Your site decides what’s possible.