Why Your Neighbor Built an ADU but You’re Running Into Problems

Your neighbor just finished building a detached ADU. The process was smooth, and now you’re ready to do the same. You’re on the same street, with the same lot size and zoning. It should be a repeatable success.

But once you get started, things don’t go as planned. Permits stall. Costs climb. Design hits unexpected roadblocks. What looked simple on the surface quickly becomes complicated.

This isn’t unusual. Lots that appear identical on paper can behave very differently in practice. Small differences in slope, access, utility lines, or how local departments interpret the code can shape the outcome often in ways that only become clear after time and money have been invested.

The Assumption

Most homeowners start with a simple question: “What’s my zoning?”

Cities encourage this. They publish zoning maps, hold workshops, and distribute flyers promoting ADU development. If both you and your neighbor are zoned R1, it seems reasonable to assume your building options are the same.

That’s where many projects go off course.

Zoning sets the legal groundwork. It tells you what types of structures are allowed and outlines basic rules like height, use, and setbacks. But it doesn’t answer the more important question: Can your lot actually support what the zoning allows?

People often mistake zoning approval for project feasibility. In reality, zoning is just one piece of a much larger puzzle. Your neighbor’s ADU might have been possible because their lot was flat, had clear utility access, or enough space along the side yard. If your lot lacks just one of those features, the same plan could be unworkable or far more costly.

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

Why Lots Can Look the Same, but Act Very Different

At first glance, your lot and your neighbor’s may seem nearly identical. You’re on the same street. You share the same zoning R1, RD2, or another single-family category. The parcels are similar in size and shape. Setback rules match, and both lots fall under the same general plan or community overlay.

On paper, everything lines up. That’s often enough for homeowners, real estate agents, and even some consultants to assume an ADU is a sure thing.

But those similarities are only part of the story.

Zoning, lot size, frontage, and plan area do matter. They set the legal framework for what’s allowed in general terms. But they don’t reveal the on-site conditions that determine whether a project can actually move forward.

Zoning maps won’t show you slope, tree locations, utility paths, or fire access routes. They don’t show recorded easements or whether your side yard is just wide enough to meet code. These small details can have a big impact on whether an ADU is possible and where it can go.

Knowing your lot appears eligible is helpful. Knowing whether it’s actually buildable is essential.

What Makes One Lot Buildable and the Other Not

Your zoning may allow an ADU, and your lot may seem similar to your neighbor’s, but whether you can actually build depends on factors zoning maps don’t show. These parcel-specific conditions often explain why one project moves forward easily while another runs into delays, redesigns, or denials.

Here are some of the most common reasons a seemingly eligible lot turns out to be unworkable:

Slope or Grade Changes Even minor elevation changes can trigger added requirements like specialized foundations, drainage systems, or structural upgrades. In hillside areas, grading plans or retaining walls may be needed. These add both time and cost. In some cases, slope alone can shrink your usable space or affect how height limits are applied.

Fire Access Most cities require a clear, code-compliant path from the street to the ADU for emergency access. That path typically needs to be at least 3 to 4 feet wide and free of obstructions. If your side yard is too narrow or the ADU is too far back, the fire department might require changes or reject the plan entirely.

Utility Conflicts ADUs must connect to sewer, water, and electricity. If service lines run beneath your planned building area or through a utility easement, you may have to reroute them. That could mean trenching across the lot or relocating a sewer lateral both of which can drive up costs. In some cases, easements may block off your preferred location altogether.

Overlay Zones and Extra Restrictions Some lots fall under additional regulations due to wildfire risk, flood zones, hillsides, or historic protections. These overlays can change the rules for height, setbacks, design, and materials. Your neighbor’s property might be just outside these zones, while yours is fully subject to them.

Existing Structures and Site Features Garages, pools, trees, or retaining walls can eat into your buildable area or interfere with required clearances. For example, a 10-foot separation between buildings might be required and that one rule could eliminate your only viable ADU location.

How to Actually Find Out What Your Lot Can Handle

By now, it’s clear that zoning alone won’t tell you whether your lot can support an ADU. So how do you find out before spending money on design, surveys, or permits?

Most homeowners start with the basics. They check the zoning map, read a brochure, or ask a contractor or Realtor. These sources might confirm that ADUs are allowed in your zone, but they rarely identify the site-specific issues that can stop a project in its tracks.

What you really need is a detailed view of your property one that goes beyond zoning to show what’s happening on your lot.

• Zoning and overlays at the parcel level Know if your lot is in a special plan area, hillside zone, historic district, or fire severity zone each of which may come with added rules.

• Recorded easements See if utility lines, shared driveways, or access agreements restrict where you can build.

• Fire access limitations Check whether your ADU site has a code-compliant path from the street before plan check flags it.

• Slope and grade changes Identify elevation shifts that could require costly grading, drainage, or structural design.

• Environmental hazards Find out if your lot falls within a flood zone, landslide area, or other regulated condition that could affect your timeline and budget.

• Utility infrastructure risks Understand where sewer, water, and electric lines run and whether they’re accessible from your proposed ADU location.

With this information, you can make smarter choices from the start about placement, design, and whether the project is even feasible.

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, two lots can have completely different outcomes.

Why? Because every property has its own physical constraints, infrastructure layout, and regulatory layers. These details often stay hidden until plans are submitted, permits are delayed, or costs start rising. By then, it’s often too late or at least expensive to adjust course.

The earlier you identify these constraints, the better. Knowing about slope, access paths, utility lines, and easements at the start can save you months of delays and thousands of dollars in redesigns or permitting issues.