Why Your Neighbor’s Lot Qualified for an ADU and Yours Might Not

You may have seen your neighbor successfully build a detached accessory dwelling unit. It appeared to go smoothly, and now you're planning to do the same. Your lot is the same size, sits on the same street, and shares the same zoning designation. On the surface, your project should follow a similar path.

But once you begin, the process becomes less predictable. Your plans stall. Design revisions multiply. Unexpected costs emerge. What initially seemed like a straightforward project turns into a complex set of challenges.

This scenario is more common than most homeowners expect. Two properties can look nearly identical on a zoning map but yield very different results when it comes to development. Factors such as slope, access width, utility location, and how local agencies interpret building and fire code can significantly affect feasibility often in ways that only become clear after time and money have already been committed.

Understanding these variables early in the process is critical. The Code Solution helps clients move beyond assumptions and into actionable planning, ensuring that entitlements align with real-world constraints.

The Assumption

Homeowners often start with a simple question: What’s my zoning?

Cities help by publishing zoning maps, holding workshops, and handing out flyers to promote ADUs. If your lot and your neighbors are both zoned R1, it’s easy to assume your options are the same.

That’s where many projects start to go off track.

Zoning sets the legal framework. It tells you what’s allowed: the type of unit, the permitted use, and some general rules. But it doesn’t answer the more important question: Can your lot actually handle what the code allows?

People often confuse zoning with feasibility. In truth, zoning is just one part of a bigger picture. Your neighbor’s ADU may have worked because of a wide side yard, a flat lot, or an ideal utility setup. If your lot has even one constraint, that same project could be impossible or much more expensive.

Zoning tells you if something is allowed. It doesn’t tell you what it will take to build it.

Why Lots Can Look the Same, But Act Very Different

At first glance, your property might seem just like your neighbor’s. Same street, same zoning R1, RD2, or another single-family designation. The parcels are about the same size and shape. Setbacks match. You're both under the same general plan or overlay zone.

On paper, everything lines up. That’s often what homeowners, agents, and even some consultants use to judge ADU potential.

But the similarities only go so far.

Zoning, lot size, frontage, and plan area do matter. They set the rules for whether ADUs are allowed and give a general outline of what might be possible. What they don’t show are the details that make or break a project.

Public zoning maps won’t show you slope, protected trees, sewer line paths, or fire lane access. They don’t include recorded easements, underground utility conflicts, or the difference between a 36-inch and 40-inch side yard. These might sound like minor details, but they can determine whether you have a buildable area or none at all.

It’s helpful to know why your lot looks eligible. But it’s far more important to understand whether it actually is.

Why One Lot Is Buildable and the Other Isn’t

Zoning may say you can build an ADU, and your lot may look like your neighbor’s on paper. But actual feasibility depends on site-specific details that zoning maps don’t show. These parcel-level conditions often explain why one project moves quickly through permitting while another runs into delays, redesigns, or denials.

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

Slope or Grade Changes Even small changes in elevation can trigger costly requirements like upgraded foundations, drainage systems, or structural engineering. In hillside areas, you might need grading plans or retaining walls, which add time and expense. Slope can also reduce buildable area or affect how height limits apply.

Fire Access Most cities require a clear path from the street to your ADU for emergency access. This path usually must be at least 3 to 4 feet wide and unobstructed. If your side yard is too narrow or the unit is set too far back, the fire department may ask for changes or reject the plan altogether.

Utility Conflicts Your ADU needs sewer, water, and power. If utility lines cross the build area or a protected easement, you might need to reroute them. That could mean trenching across the lot or moving a sewer lateral, both of which are expensive. A utility easement might block you from building in that part of the yard entirely.

Overlay Zones and Special Restrictions Some lots fall under extra rules due to things like fire risk, flooding, or historic status. These overlays often mean tighter limits on height, setbacks, materials, or design. Even if your neighbor lives nearby, they might not face the same restrictions.

Existing Structures and Obstacles What’s already on your lot matters. Detached garages, pools, large trees, or retaining walls can limit where you can build. A required 10-foot distance between buildings could rule out your preferred spot. Sometimes, these physical constraints leave no buildable area at all.

How to Actually Find Out What You Can Build

By now, it’s clear that zoning is only part of the picture. So how do you figure out whether your lot can truly support an ADU before you spend money on plans, surveys, or permits?

Most homeowners start with the basics. They look at the city’s zoning map, skim a brochure, or ask a contractor or Realtor. These sources might confirm that ADUs are allowed in your zone, but they rarely uncover the physical or regulatory issues that could block your project.

What you really need is a parcel-specific view of the conditions that affect buildability. 

• Zoning and overlays at the parcel level Know your exact zone and whether you're affected by hillside rules, fire zones, historic overlays, or community plans.

• Recorded easements Find out if utility lines, shared driveways, or right-of-way easements restrict where you can build.

• Fire access limitations See if there’s a code-compliant path from the street to your ADU site before the fire department flags it during review.

• Slope and grade analysis Identify elevation changes that could trigger grading, retaining walls, or reduce usable space.

• Environmental constraints Check for floodplains, landslide zones, or other hazards that require special permits or studies.

• Infrastructure and utility risks Understand where your sewer, water, and electric lines run and whether connecting to them is realistic from your proposed build site.

With this level of detail, you can plan smarter. You’ll know where the unit can go, how to design it, and whether it’s worth moving forward at all.

Final Thoughts: Same Zoning, Different Reality

Your neighbor may have built an ADU, but that doesn’t mean you can. Even on the same street, under the same zoning code, two lots can have very different outcomes.

That’s because zoning only tells part of the story. Each property comes with its own physical conditions, overlays, and infrastructure issues. These differences often stay hidden until it’s too late when plans are drawn, permits are submitted, or costs start piling up. By then, you may be stuck redesigning your project or walking away from an idea you thought was viable.

The sooner you uncover those constraints, the better. Knowing your lot’s slope, access paths, utility layout, and easements up front can save you months of delay and thousands in wasted fees.

Zoning gives you the green light.

 Your site conditions decide how far you can go.