The term "building area" often appears when planning a custom-built home. It is easy to confuse with total floor area or land area, and if you do not understand it correctly, it can lead to problems. It is important to understand it together with the building coverage ratio and floor area ratio.
What Is Building Area?
Building area is a term under Japan's Building Standards Act that refers to the area of a building when viewed directly from above (horizontal projection area). It shows how much of the site is occupied by the building, which consists of the roof, columns, and walls. An uncovered courtyard or parking space is not included, but a covered carport is included in the building area. In general, the first-floor area is treated as the building area.
What Is Included in Building Area
Protruding portions such as balconies, eaves, and porches are not included in the building area if they extend 1 meter or less. For portions extending more than 1 meter, the area beyond a 1-meter deduction from the tip is added to the building area. If walls or columns are present, the portion may be included even when it extends 1 meter or less.
What Is the Difference Between Building Area, Total Floor Area, and Land Area?
| Term | Meaning |
|---|---|
| Building area | The footprint seen from directly above the building (typically the first-floor area) |
| Total floor area (gross floor area) | The sum of floor areas on all floors (1st floor + 2nd floor + ...) |
| Land area (site area) | The total area of the land on which the building stands |
| Building floor area | Used as a synonym for total floor area in real estate listings and similar materials |
What Is the Building Coverage Ratio? Calculation and Limit
The building coverage ratio is "the percentage of land area occupied by the building area", and each zoning area has its own maximum limit.
Building coverage ratio (%) = Building area ÷ Land area × 100
Example: If the land is 100㎡ and the building is 60㎡, the building coverage ratio is 60%. If the local limit is 60%, the design is right at the upper limit. A building that exceeds the limit cannot be constructed.
Relaxation Measures for the Building Coverage Ratio
In fire prevention zones and quasi-fire prevention zones, the building coverage ratio may be relaxed. This often applies to areas near stations or along major roads where buildings are densely packed.
What Is the Floor Area Ratio? Difference From the Building Coverage Ratio
The floor area ratio is the maximum "ratio of total floor area to land area". Even if a building complies with the building coverage ratio, exceeding the floor area ratio is still a violation.
Floor area ratio (%) = Total floor area ÷ Land area × 100
For the impact of a carport on the building coverage ratio, please also see Key points on installing a carport and the building coverage ratio.
Frequently Asked Questions (FAQ)
Q1. What is the difference between building area and total floor area?
Building area is the "footprint seen from directly above" and is typically the first-floor area. Total floor area is the "sum of floor areas on all floors," so in buildings with two or more floors, it is larger than the building area.
Q2. What happens if I build beyond the building coverage ratio?
It becomes a non-compliant building and may be subject to administrative guidance and corrective orders. It can also create problems when selling the property, so prior confirmation is essential.
Q3. Can I check the building coverage ratio myself?
You can search for "the area where you want to live + zoning district," or confirm it with a real estate company or your municipal building guidance division.
Q4. Is a balcony included in the building area?
If the projection length is 1 meter or less, it is not included. Any portion exceeding 1 meter is counted toward the building area according to a prescribed calculation method.
