Determining the floor area for the purposes of regulations or construction in Japan, can be complex. Here you will find a brief introduction to the most common definitions you will need to be aware of, when planning in Japan.
Floor area calculation system in Japan
A 敷地面積 Shikichi menseki - site area; lot area; lot size
B 容積対象床面積 Youseki taishou menseki (容積対象面積)
Defined by the center line of the outer wall or column, included technical spaces on the roof ペントハウス(PH) and ducts パイプスペース(PS). Excluded: elevator (エレベーター室), open spaces, external staircase. It is used for calculating the floor area ratio 容積率.
B1 容積率 yousekiritsu － floor area ratio (FAR)
C 建築面積 Kenchiku menseki －building coverage area
Defined by the center line of the outer wall or column, open areas are not included.Therefore, balconies, verandas, and eaves are not included in the building area, till the depth of 1 meter (for example, if the total width is 1 meter and 40 centimeters, 40 centimeters are included). The "bow windows" at a height of 30 cm or more from the floor, and less than 50 cm out from the wall are not counted. Balconies and verandas surrounded by walls on three sides are included.
The building coverage area 建築面積 is defined by the Building Standards Law, and it is used for calculating the building coverage ratio 建蔽率.
C2 建蔽率 or 建ぺい率 Kenperitsu (BCR)% = building coverage area / site area × 100
D 容積対象外床面積 ＝ 駐車場 ＋ エレベーターホール ＋ 屋内廊下 ＋ 屋内階段
Yōseki taishō-gai tokomenseki = chūshajō + erebētāhōru + okunai rōka + okunai kaidan
Excluded Floor Area = Parking + Elevator + Indoor Hallway + Indoor Staircase
E 延べ床面積 nobeyukamenseki - total floor area (延床面積)
延べ床面積＝ 容積対象床面積 ＋ 容積対象外床面積
Nobe tokomenseki = yōseki taishō tmenseki + yōseki taishō-gai tokomenseki
F 施工床面積 ＝ 延床面積 ＋ 外廊下 ＋ 外階段 ＋ バルコニー
shikō yuka menseki ＝ Nobe tokomenseki + soto rōka + tokaidan + barukonī
Building Coverage Ratio (BCR) 建蔽率 Kenperitsu and the floor area ratio (FAR) 容積率 yousekiritsu
Each zone restricts the building size and density by prescribing the numbers used to calculate the buildable area 建蔽率 Kenperitsu and 容積率 yousekiritsu (Building Coverage Ratio - BCR and floor area ratio - FAR).
The purpose of the BCR 建蔽率 or 建ぺい率 Kenperitsu is to limit building density, preventing fires from spreading between adjacent buildings; ensuring light and air penetration; controlling urban character.
Kenperitsu (BCR)% = building coverage area / site area × 100
Japan has extensively adopted the floor area ratio in the zoning system since 1970. Some say that it has deteriorated the skylines and building lines in Japanese cities, others claim that it has protected the residential environments. The floor area ratio (FAR) – yousekiritsu 容積率 – sets the maximum possible floor area, totalled across all floors and expressed as a percentage of the site area. In other countries Floor Area ratio is sometimes called floor space ratio (FSR), floor space index (FSI), site ratio or plot ratio. The difference between FAR and FSI is that the first FAR a ratio, while the FSI is an index. For example 250% FAR is 2.5 FSI.
Yousekiritsu (FAR)% = total floor area / site area × 100
When constructing buildings across multiple zones with different prescribed ratios, in Japan, BRC and FAR should be calculated with a weighted average.
Where to get informations
This guide is by no means complete, is just a quick overview of aspects of Japanese Building Standard Law. It is also a way l to show how things can be done differently.
yousekiritsu － floor area ratio (FAR)
Kenperitsu = building coverage area (BCR)