How I Check for Illegal Additions Before Closing on a Property

Three years ago I almost bought a duplex with a second-floor sunroom that didn’t show up on a single city record. The seller called it a bonus room. The city called it an unauthorized addition waiting to become my problem. That near-miss cost me nothing in the end, but it’s the reason I now spend at least 90 minutes at city hall before I ever sign a purchase agreement.

Illegal additions, or what’s often called mudan zoutetsu in Japanese property circles, are more common than most first-time investors expect, especially in anything built before 1990. Out of the 9 properties I currently own, I walked away from 4 other deals and negotiated price cuts on 2 more purely because of what I found during this check. It’s the single most valuable habit I’ve built into my buying process.

Why This Matters More Than It Looks Like It Does

An unpermitted addition isn’t just a paperwork technicality. Lenders can refuse to finance a property once they spot a mismatch between the registered floor area and the actual structure, which can kill a deal at the worst possible moment. Fire insurance can be voided on the affected portion of the building. And resale value typically drops 10 to 20 percent once a buyer’s inspector flags the same issue you’re flagging now. I’ve also seen a demolition estimate for a small illegal extension come in around 800,000 yen, money that goes straight into removing something the previous owner built without asking anyone. On top of that, some ward offices will refuse to issue a routine certificate for the whole building until the violation is cleared, which can hold up a future sale or refinance for months.

Step One: Comparing the Floor Plan to the Building Registry Ledger

The first stop is always city hall. I request a copy of the kenchiku daicho, the official building registry ledger, which usually costs around 300 yen per copy and takes about 15 minutes to pull. I lay that next to the floor plan the agent gave me and compare total floor area in square meters, room by room if the listing includes dimensions. On that sunroom duplex, the ledger listed 78.5 square meters. My tape measure on-site came out to 92 square meters. A 13.5 square meter gap like that is not a rounding error, it’s a room that officially doesn’t exist.

Step Two: Walking the Property With a Tape Measure and a Camera

Paperwork only tells half the story, so I walk every property with a 5.5 meter tape measure and my phone camera before I make an offer. Things I specifically look for:

• Enclosed balconies or verandas that look newer than the main structure
• Carports or storage sheds attached directly to the house rather than freestanding
• Roof extensions or dormers with siding that doesn’t match the original exterior
• Interior walls that create a room shape not reflected in the listed floor plan
• Any second staircase or separate entrance suggesting an unregistered unit

I also pull up the property address in an old satellite image archive and compare the roofline to what’s on the ground today. If a structure appears in a photo from 8 or 10 years ago that isn’t on the current building ledger, that’s my cue to ask the agent directly rather than assume it’ll sort itself out later.

Step Three: Checking the Confirmation and Completion Certificates

Every legally built structure in Japan should have a kakunin shinsei, the building confirmation issued before construction, and ideally a kensa kanryo sho, the completion inspection certificate issued after. I ask the seller’s agent for both before I go further, and I’ve learned not to accept a verbal it’s probably somewhere. Out of my own last 9 purchases, only about 60 percent had paperwork that fully matched the physical layout on the first pass. The rest needed follow-up questions, and two needed a formal correction filed with the city before closing.

What I Do When I Actually Find One

Finding an illegal addition doesn’t automatically mean I walk away, though I have done that 4 times when the numbers didn’t work. My options are usually to negotiate the purchase price down to cover future removal or legalization costs, to request the seller handle the correction before closing, or to simply pass on the deal. On one property, documenting a 13 square meter unauthorized extension got me a 1.2 million yen reduction off the asking price, which more than covered the eventual cost of bringing it up to code. For anything involving a significant discrepancy, I bring in a kenchikushi, a licensed architect, rather than trying to judge the legal exposure myself. A proper consultation usually runs somewhere between 30,000 and 50,000 yen, which is a small price compared to guessing wrong.

My Pre-Closing Checklist

After going through this process more times than I’d like to admit, here’s the short version I actually follow on every deal now:

• Pull the building registry ledger and compare total floor area
• Measure the actual structure with a tape measure, room by room
• Compare current satellite or street-view images against older ones
• Request both the confirmation and completion certificates in writing
• Flag any gap over roughly 5 square meters for a professional opinion
• Get any negotiated price reduction or seller commitment in writing before signing

That 90 minutes at city hall has saved me from at least one genuinely bad purchase and put more than a million yen back in my pocket on another. That’s the approach I use across all 9 of my properties, and I don’t skip it no matter how good a deal looks on paper.

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