How To Check If A Japanese Property Has Illegal Construction

What "illegal construction" means, and how it differs from a legally grandfathered building

Japan's Building Standards Act (建築基準法, Act No. 201 of 1950) requires an owner planning new construction, or a major repair, remodel, or extension of certain buildings, to first apply for and receive 建築確認 (kenchiku kakunin, "building confirmation") — a pre-construction review confirming the plan meets the applicable rules, including the site's building coverage ratio (建ぺい率), floor area ratio (容積率), and road-frontage and setback requirements (Building Standards Act Art. 6). This requirement is not universal for every project: which buildings need it depends on factors like size, number of stories, location, and use, and separately, an extension, alteration, or relocation adding no more than 10 square meters of floor area is generally exempt from the confirmation procedure itself outside fire-prevention and quasi-fire-prevention districts (Art. 6, para. 2) — though the work still has to meet the underlying building standards even without going through confirmation. New construction, by contrast, generally needs confirmation regardless of size within a city-planning area. Once a project that does require confirmation is finished, the owner must apply for a completion inspection, and if the finished building and site actually match what was confirmed, the inspector issues a 検査済証 (kensa zumi shou, "completion inspection certificate") (Building Standards Act Art. 7).

違反建築物 is a building that was illegal from the start — built without confirmation it should have had, or built in a way that departed from the confirmed plan — or one that became illegal later because of unauthorized work, such as an addition or renovation that needed confirmation but never got it. This is a genuinely different situation from 既存不適格建築物 (kizon futekikaku kenchikubutsu, an "existing non-conforming building"): a building that fully complied with the rules in force when it was confirmed, but no longer conforms because the zoning or the law itself changed afterward — a tightened floor area ratio for the district, for example, or a revised road-width rule. An existing non-conforming building is not a violation and is not normally penalized for the mismatch; a genuinely illegal building is.

A 検査済証 only tells you the building matched its confirmed plan at the moment of that inspection — it is not an ongoing guarantee. A building with a valid, original 検査済証 can still become illegal later if an unauthorized addition or alteration was made afterward, so having the certificate on file is not proof the building is compliant today; it only rules out one specific failure mode (that the building was never confirmed compliant to begin with). Without it, that original compliance can't be established at all, which is why a missing 検査済証 raises the question without settling it — it makes an illegal building impossible to rule out, but does not itself prove one exists. Either way, what actually decides the current answer is comparing the building as it stands today against the confirmed plan and the rules that applied when any given piece of work was done — not the mere presence or absence of a certificate.

Why the registration record won't tell you this

Japan's property registration system, run by the Legal Affairs Bureau under the Real Property Registration Act (不動産登記法), exists to record a property's physical description and ownership history, not to certify compliance with the Building Standards Act. A building's registration record can be filed showing its floor area, structure, and number of stories even if the building itself violates building code — professional guidance from land and house surveyor associations is direct on this point: registration and legal compliance are handled by entirely separate systems, and a building can be registered regardless of a violation, with one narrow exception — if the local authority has already issued an order to remove the building, that order blocks new registration.

In practice this means ordering a building's registration record and finding a clean, unremarkable entry tells you nothing about whether the building complies with current building code. It tells you what was filed about the building's physical description — which is still worth comparing against what you can see on site, since a mismatch is one of the more reliable signs that unauthorized work happened. A related but distinct question is whether a property has enough road frontage to be rebuilt at all if the existing structure were ever removed — see How To Check If A Japanese Property Is Rebuildable for that separate check.

How illegal construction actually happens on real properties

The most common patterns are additions or alterations made without a new building confirmation: enclosing what was originally an open balcony, converting a garage or carport into a living room, adding a room on the roof, or extending a structure's footprint. Any of these can push a building over its site's building coverage ratio or floor area ratio limit, or bring it too close to a boundary or road, without the owner ever going back for a fresh confirmation. It is worth distinguishing that path from the grandfathered case above: a coverage or floor-area excess that came from a zoning change after a compliant original confirmation is 既存不適格, not a violation — the difference is whether the excess came from a change in the rules, or a change made to the building itself without approval.

It also helps to know that a meaningful share of existing buildings were never issued a 検査済証 in the first place, simply because completion inspection compliance has improved substantially over time rather than always being near-universal. According to MLIT data, the nationwide completion inspection rate reached roughly 89% in fiscal 2011, having climbed past 90% only around 2008; rates in earlier decades were lower and varied more by region and building type. That history matters for older houses and akiya in particular — a missing 検査済証 on an older property is common enough on its own that it should prompt a closer look, not an assumption that the building must be illegal.

Here is a practical way to check before you get attached to a listing:

1. Ask the seller or agent directly for the 検査済証 and the original 建築確認済証. This is the first and most direct check — many properties, especially older houses and akiya, will not have these on hand even if the building is entirely legitimate.
2. If those certificates are missing, request a 台帳記載事項証明書 from the municipal building department. This certified extract of the confirmation and inspection ledger (issued by the building permit authority, 建築主事, covering the property) states factually whether and when the building received confirmation and completion inspection — useful even when the original paper certificates were lost. Procedures vary by municipality, so check with the building department (建築指導課) for the property's specific city or ward.
3. Ask to view the 建築計画概要書 on file. This building plan overview is kept by the local building authority until the building is demolished or removed, and in most municipalities can be viewed without needing to show a specific interest in the property (Building Standards Act Art. 93-2 and its Enforcement Regulations). The standard form covers summary attributes — use, structure, floor area, height, and number of stories — plus a vicinity map and a site plan showing the building's footprint and position on the lot; it does not include the detailed room-by-room floor plans from the original confirmation application, so for comparing an enclosed balcony or an added room against the original design, ask the architect, builder, or municipal building department whether the underlying confirmation application drawings are still available.
4. Order the building's registration record and compare its recorded floor area and structure to what's actually on site. See How To Order A Japan Property Registration Record In English for the ordering workflow. A discrepancy — an enclosed space, an extra room, or a floor area larger than what's on file — is a practical sign that work happened without going back through confirmation, though remember the registration record itself cannot confirm or rule out a violation on its own.
5. Check the site's building coverage ratio and floor area ratio limits for its zoning district, and do a rough sanity check against the building's footprint and floor area. See How To Check Zoning And Hazard Maps In Japan for how to look up these limits. This won't replace a professional survey, but an obvious mismatch is worth raising before you go further.
6. If you find a discrepancy, don't stop at the zoning classification — check the full history of the rules that applied to the specific piece of work, and when that work was actually done. A zoning reclassification is only one way a building can end up grandfathered; a coverage ratio, floor area ratio, or other building standard can also be revised without the zoning classification itself changing, and the structure may have been altered more than once at different times. Whether you're looking at a legally grandfathered building or a genuine violation depends on comparing each piece of construction against the rules actually in force when it was done — not a single before/after check against one zoning-change date.
7. Ask your lender directly whether they will finance the specific property, rather than assuming. Lenders are commonly cautious about a building with an unauthorized addition or a confirmed violation, though some will still finance a modest coverage or floor-area excess, and a legally grandfathered mismatch is generally easier to finance than one caused by unpermitted work.
8. If you need to establish current compliance status and no 検査済証 exists, ask a licensed architect or a designated confirmation inspection agency (指定確認検査機関) about a building code compliance survey under MLIT's current guideline for existing buildings. This produces a survey report some lenders and municipalities will accept as evidence for a future addition or renovation — but it is not a substitute 検査済証, and it does not retroactively legalize an actual, confirmed violation.
9. Match anything unresolved to the right professional. A licensed architect (建築士) is who can actually assess whether specific construction complies with the Building Standards Act and distinguish a genuine violation from a grandfathered mismatch. For the legal consequences of a corrective order, or how a violation should affect the purchase itself, bring in an attorney (弁護士). A judicial scrivener (司法書士) is still the right person for the registration and closing side of the transaction, but is not a substitute for either of the above on a genuine compliance or corrective-order question.

The most common mistake is treating a clean registration record as proof the building is legal — the registry and the Building Standards Act are separate systems, and a building can be duly registered while sitting in violation of building code. A close second is assuming a missing 検査済証 by itself proves an illegal building, when it is also consistent with an older property that predates today's near-universal completion inspection rate.

What the registration record can and cannot tell you here:

• What it shows: the building's recorded floor area, structure, and number of stories as filed — useful for comparing against what is actually built on site.
• What it does not show: whether the building complies with the Building Standards Act's building coverage ratio, floor area ratio, road-frontage, or setback rules, or whether the confirmed plan was actually followed during construction. Registration and legal compliance are handled by separate systems, and a building can generally be registered even in violation — the one exception is that a local authority's demolition order against the building blocks new registration.
• How to order the record: On Japan Property Research, order the building's registration record for ¥1,500 to check the recorded floor area and structure — pair it with a municipal building-department check (the 台帳記載事項証明書 and 建築計画概要書) for the legal-compliance side, since that sits outside anything a registry search can answer.

The consequences of a genuine violation are real, though not automatic. Under Building Standards Act Art. 9, the local administrative authority (特定行政庁) can order the owner to stop work, or to remove, rebuild, or repair a building found in violation; ignoring such an order can carry a penalty of up to three years' imprisonment or a fine of up to ¥3,000,000 — though that penalty attaches to defying the corrective order itself, not automatically to owning an illegal building, and enforcement action is not triggered just because a violation technically exists. The more immediate practical risk for a buyer is financing and resale: lenders are commonly reluctant to finance a building with an unauthorized addition or a confirmed violation, and a future buyer's own due diligence will surface the same gap you would be inheriting from the seller. Our Japan property due diligence checklist covers where this check fits alongside ownership, mortgages, and rebuildability.

FAQ

Does a Japanese property's registration record show whether a building is illegal? No. The registration record shows the building's filed floor area, structure, and number of stories, but registration and compliance with the Building Standards Act are handled by separate systems. A building can be registered even if it violates building code, with one exception: registration is blocked once a local authority has issued a demolition order against it.

What's the difference between illegal construction and a legally grandfathered building? An illegal building (違反建築物) either skipped a building confirmation it was actually required to have, departed from its confirmed plan, or was altered afterward by work that needed confirmation but never got it, or that violated a building standard regardless of confirmation. A later alteration that fell within the Art. 6(2) exemption (a qualifying extension of no more than 10 square meters outside a fire-prevention or quasi-fire-prevention district, say) and still met the substantive standards does not make the building illegal on its own. A legally grandfathered building (既存不適格建築物) is a different case entirely: it fully complied with the rules when it was confirmed, but a later change in zoning or the law itself made it no longer conform — that mismatch is not a violation and is not normally penalized.

Is a missing 検査済証 proof that a building is illegal? Not by itself. It means you can't point to direct proof the building was confirmed compliant at completion, but a meaningful share of existing buildings, especially older ones, were never issued a completion inspection certificate at all — the nationwide completion inspection rate only climbed past 90% around 2008. Treat a missing certificate as a reason to check further, such as a 台帳記載事項証明書 from the municipal building department, not as confirmation of a violation.

Can I get a mortgage on a property with illegal construction? It depends on what caused the issue and how much it exceeds the limit. Lenders are commonly cautious about a confirmed violation or an unpermitted addition, though some will still finance a modest building-coverage or floor-area excess, and a legally grandfathered mismatch is generally easier to finance than one caused by unauthorized work. Ask your specific lender rather than assuming either way.

Can illegal construction be fixed after I buy? Sometimes, depending on the violation and what a licensed architect or the municipal building department says is required — this can mean removing the unauthorized portion, or in some cases going through a building code compliance survey to establish current status for a future renovation. It is not a guaranteed fix, and it is not free, so treat it as a real cost to factor in before you commit rather than something to resolve after closing.

Related guides:

• How To Check If A Japanese Property Is Rebuildable
• How To Check If A Japanese Property Has An Unregistered Building
• How To Check If A Japanese Property Meets Earthquake Standards
• How To Check Zoning And Hazard Maps In Japan
• How To Read A Japanese Property Registration Record
• Japan Property Due Diligence Checklist
• How To Order A Japan Property Registration Record In English

• Building Standards Act (建築基準法), Article 6 — Building Confirmation — Ministry of Justice, Japanese Law Translation
• Building Standards Act (建築基準法), Article 7 — Completion Inspection and Inspection Certificate — Ministry of Justice, Japanese Law Translation
• Building Standards Act (建築基準法), Article 9 — Corrective Orders for Illegal Buildings — Ministry of Justice, Japanese Law Translation
• 10㎡以下の増築では確認申請は不要か～床面積が増えない場合でも申請が必要なケースとは～ — Bureau Veritas Japan — confirms the Art. 6, para. 2 exemption from the confirmation procedure for a ≤10㎡ extension/alteration/relocation outside fire-prevention and quasi-fire-prevention districts, and that the underlying building standards still apply even without confirmation
• 建築計画概要書（第一面～第三面）様式 — Ministry of Land, Infrastructure, Transport and Tourism (MLIT) — the official form, confirming its contents are summary attributes plus a vicinity map and site plan (配置図), not detailed room-by-room floor plans
• 既存建築物の現況調査ガイドライン（第２版） — Ministry of Land, Infrastructure, Transport and Tourism (MLIT), March 2025 — the current guideline (effective April 1, 2025) for surveying an existing building's Building Standards Act compliance status when no 検査済証 exists, superseding MLIT's 2014 guideline
• 【国土交通省】「既存建築物の現況調査ガイドライン」の公表について — Zentaku (全宅連, National Federation of Real Estate Associations) — confirms the guideline's publication and the transition from the prior 2014 guideline
• 資料４ 効率的かつ実効性ある確認検査制度等のあり方の検討 — Ministry of Land, Infrastructure, Transport and Tourism (MLIT) — official data on completion inspection issuance and the roughly 89% fiscal-2011 completion inspection rate
• 確認済証と検査済証はいつからある？義務化の時期をわかりやすく解説！ — 建築基準法のトリセツ — background on how the completion inspection rate rose over time and only became consistently high after the late 2000s
• 違反建築物と既存不適格建築物についてまとめた — Iqra Fudosan — practical explanation distinguishing 違反建築物 from 既存不適格建築物
• ヒント3 - 既存不適格建築物と違法建築物の違い — リフォネット (Refonet, a renovation-industry information site) — explains the role of 検査済証 as proof of original compliance
• 違法建築の建物登記は可能か？ — 岐阜県土地家屋調査士会 (Gifu Prefecture Land and House Surveyors Association) — confirms that registration and Building Standards Act compliance are separate systems, and that registration is blocked only once a demolition order has been issued
• 企業法務事例コラム│これだけは絶対するな！！建築の法令違反に対する処分と罰則 — 弁護士法人DREAM (law firm) — confirms the Art. 9 corrective-order framework and the penalty for violating a corrective order
• 建蔽率、容積率オーバー物件のメリット・デメリット、融資、罰則について解説 — URUHOME — practical discussion of financing difficulty for coverage-ratio and floor-area-ratio violations, and the distinction between a zoning-driven and an unpermitted-work-driven excess