When purchasing land and constructing a building, one important legal indicator you must always verify is the "building coverage ratio." Proceeding with plans without understanding the building coverage ratio can result in being unable to build the structure you envisioned. This article explains the definition, calculation method, relaxation conditions, and region-specific examples of the building coverage ratio from a real estate regulation perspective.
What Is the Building Coverage Ratio? Definition and Purpose
The building coverage ratio is an indicator that expresses the proportion of the building footprint (the area of the building as seen from directly above) relative to the lot area. Upper limits are set by local governments (designated administrative agencies) for each zone, with 40–60% being common in residential use zones and up to 80% in commercial and industrial zones.
Reasons the building coverage ratio exists:
- To ensure ventilation and sunlight between buildings
- To prevent the spread of fire
- To maintain a pleasant townscape in the area
Formula: Building Coverage Ratio = Building Footprint ÷ Lot Area × 100
Example) For a lot area of 120㎡ and a building footprint of 75㎡: 75 ÷ 120 × 100 = 62.5%
Understanding the Difference from the Floor Area Ratio
The floor area ratio is the proportion of a building's total floor area (sum of all floors) relative to the lot area. While the building coverage ratio restricts "horizontal spread," the floor area ratio restricts "three-dimensional scale."
Formula: Floor Area Ratio = Total Floor Area ÷ Lot Area × 100
Even if a building falls within the building coverage ratio, it cannot be built taller if it exceeds the floor area ratio limit, making it essential to check both simultaneously.
3 Cases Where Building Coverage Ratio Relaxations Apply
For lots that meet certain conditions, a 10% additive relaxation to the building coverage ratio upper limit is permitted.
① Corner Lots
For "corner lots" where two roads each border the lot by 2 meters or more, the building coverage ratio is relaxed by 10%. Cases where a park, plaza, river, or rice paddy (rather than a road) borders the lot may also qualify for relaxation, but the criteria differ by municipality, so prior confirmation is necessary.
② Fire-Resistant Buildings in Fire Prevention Zones and Quasi-Fire Prevention Zones
In fire prevention zones, the building coverage ratio is relaxed by 10% exclusively for fire-resistant buildings (or fire-spread-prevention buildings). In quasi-fire prevention zones, the relaxation applies if the building qualifies as any of the following: fire-resistant, fire-spread-prevention, quasi-fire-resistant, or quasi-fire-spread-prevention.
③ Lots Flanked by Two Roads
The relaxation applies when two roads each border the lot by 2 meters or more and the lot satisfies the "Building Standards Act Enforcement Rules" established by the designated administrative agency.
Building Coverage Ratio Comparison Among Major Cities
| City | Area Example | Building Coverage Ratio |
|---|---|---|
| Setagaya Ward | Seijo / Todoroki (upscale residential) | 40% |
| Setagaya Ward | Akatsutsumi / Tamagawa (commercial / apartment) | 60–80% |
| Nagoya City | Marunouchi (commercial zone) | 80% |
| Nagoya City | Fujimidai (exclusive residential zone) | 40% |
| Chiba City | General urban area | 50% (post-revision) |
Frequently Asked Questions (FAQ)
- Q. Can you briefly explain the difference between the building coverage ratio and the floor area ratio?
- A. The building coverage ratio indicates "how much of the land area can be covered by the building footprint (horizontal)," while the floor area ratio indicates "how large the total floor area of the building can be relative to the land (three-dimensional)."
- Q. Where should I check whether the corner lot relaxation applies?
- A. It is recommended to confirm in advance with the designated administrative agency or building inspection institution that has jurisdiction over the lot. You can also check by searching for "Building Standards Act Enforcement Rules" on your municipality's website.
- Q. What happens if a building is constructed that exceeds the building coverage ratio?
- A. Construction cannot proceed because building confirmation will not be granted. If a building already exists in violation, it becomes an illegal structure and is subject to a corrective order.
- Q. Can I build as many stories as I want as long as I stay within the building coverage ratio?
- A. No. Because there is also a floor area ratio limit, exceeding the floor area ratio prevents building taller even within the building coverage ratio. Checking both is mandatory.
