Japanese building and regulatory terms

The output of koyu and the bundled examples use terms from Japanese architectural practice and the Building Standards Act. This page is for looking them up. The words of koyu itself are in the glossary.

What is written here is not an explanation of the regulation but of how koyu treats the term. Which areas count, which are exempt, and where relaxations apply differ by jurisdiction, and koyu does not go there.

Area and ratios

Term	Definition	How koyu treats it
延床面積 gross floor area	The sum of the floor areas of every storey	The Total from stats and Total floor area from site. Derived as the wall-centerline area of spaces that have a region and a level and are neither void, exterior, nor semi-outdoor
建築面積 building footprint area	The horizontal projected area of the building seen from directly above	Building footprint from site, derived as the union of the horizontal projections of the interior spaces. Canopies and balconies are handled roughly
敷地面積 site area	The horizontal projected area of the site	Site area from site. Derived by the shoelace formula if a polygon exists, otherwise as the union of the spaces within the site and the building projection. Write a survey figure in zone /site … area: and the two are reconciled
建蔽率 building coverage ratio	Building footprint ÷ site area. A term of Article 53 of the Japanese Building Standards Act	site derives it as building coverage ratio. It is never checked against a permitted limit
容積率 floor area ratio (FAR)	Gross floor area ÷ site area. A term of Article 52	site derives it as floor area ratio. Neither the restriction by front-road width nor the exemptions (parking, common corridors) are applied
専有面積 net (exclusive) floor area	The part a unit owner uses alone	The aggregation of use:exclusive. Bundle it in a zone and a dwelling divided into rooms counts as one dwelling alongside one that is not
共用部 common area	Corridors, stairs, lift lobbies — what everyone uses	The aggregation of use:common
レンタブル比 rentable ratio	Lettable area as a share of gross floor area	The percentage for use:rentable in the By use: line of stats
壁芯 wall centerline	The middle of the wall thickness. Japanese area measurement is based on it	Areas and wall segments are all centerline-based. The thickness t is split evenly to either side

Height and section

Term	Definition	How koyu treats it
FL floor level	The finished floor height of a storey	The second positional of level L2 4000
階高 floor-to-floor height	From the FL of one storey to the FL of the one above	Never declared. Derived as the difference in z between levels, and printed by levels as storey height
天井高 ceiling height	From finished floor to finished ceiling	h: on a level. It can be overridden per space
床組み厚 slab thickness	The floor structure with its finishes and plenum	slab: on a level. check verifies that it plus the ceiling height fits inside the floor-to-floor height
懐 plenum	The space above the ceiling	Never written. Floor-to-floor minus ceiling height minus the slab above appears in levels as left over
矩計 section stack-up	The drawing that cuts the building vertically and traces the heights of each part	The textual section that levels prints
基準階 typical floor	A storey whose plan repeats	Begin the path /L3..L10/ and write it once
例外階 exception floor	A storey that differs from the typical floor	Write only the difference, in a separate layer
地下階 basement storey	A storey below ground level	level B1 -3700 … underground:1. Never inferred from a negative z — it is a declaration

Site and landscape

Term	Definition	How koyu treats it
接道 road frontage	The length over which a site adjoins a road. Article 43 requires at least 2 m	Derived as the sum of boundary segment lengths between spaces under the site zone and exterior spaces carrying road:<width>. An external wall of the building facing the road is not frontage
前面道路 front road	The road the site adjoins	space /road-s exterior road:22000. The width is written in millimetres
隣地境界 neighbouring boundary	The line between this site and the next	An edge of the polygon, or a boundary between a space within the site and an exterior such as /out/n
隅切り corner cut	Cutting the corner off a site at an intersection	Write a line indented under the boundary
歩道状空地 pedestrian setback	Part of the site used as though it were a pavement	Placed as a real exterior space on L1
塀・フェンス wall, fence	A structure enclosing the site	A value in the boundary's spec vocabulary. Add air:1, since it blocks neither air nor light

Rooms and use

Term	Definition	How koyu treats it
居室 habitable room	A room used continuously for living, working or leisure. A term of Article 2(4)	Never inferred from the type. Whether it is in scope for daylight is written as daylight:1 or daylight:0, because a change of use changes the judgement for a room of identical dimensions
採光 daylighting	The effective opening area a habitable room requires. Article 28 asks for at least 1/7 of the floor area (Order Article 19 lists what is in scope)	light roughly checks that effective window area ≥ floor area / 7. No correction factors are used
半屋外 semi-outdoor	A part open to the air but roofed or enclosed (a balcony, an external stair, a terrace)	Derived, not declared. A space with a region carrying an open or air:1 boundary with the outside becomes one, drops out of the interior floor area, and is reported separately
戸境壁 party wall	The wall between two dwellings	Written with the boundary's spec and sound: (acoustic grade)
内廊下 interior corridor	A common corridor running inside the building	A corridor-typed space. It does not become semi-outdoor unless it faces the outside
防火区画 fire compartment	A range subdivided to contain a fire	Carried by fire: (a fire rating) on the boundary and by the door assets (fire:特定防火設備 and the like). Whether a compartment is properly formed is not judged
バックヤード back of house	The operational part the public does not see	A backyard-typed space. Whether a common corridor can reach vertical circulation without passing through one is what validate looks at

Vertical circulation

Term	Definition	How koyu treats it
蹴上 riser	The height of one step	Never written. Derived from the level difference and the usual range; runs prints risers of 176mm
踏面 tread	The depth of one step	Never written. Derived
踊り場 landing	The flat part along a stair	Never written. It appears on the form side as the remainder of the step division
折返し階段 return stair	A stair that turns through 180°	form:return on the space
直進階段 straight stair	A stair rising in one direction	Write no form: (the default)
斜路 ramp	A sloped route, such as for vehicles	ramp:E form:return slope:6 on the space. slope: declares a limit; the actual slope comes from the level difference over the derived going
エスカレーター escalator	A moving stair	escalator:N on the space. The connecting boundary is type:stair (passable)
昇降機 lift	An elevator	lift:1 on the space. The connecting boundary is type:shaft (not passable) — nobody walks a shaft, so as circulation it is a different question
コア core	The zone gathering stairs, lifts, lavatories and risers	A run of spaces. Begin the path /B2..L19/ and every storey of it is written once
PS / EPS pipe and electrical risers	Vertical service shafts	shaft-typed spaces

Doors and openings

Term	Definition	How koyu treats it
建具 door or window as a product	The opening parts	The opening (door / window) and the asset holding its type
掃き出し窓 full-height window	A window reaching the floor	window w:2600 h:2200 sill:0. If it is walked through as well, write a separate door
腰窓 sill window	A window starting at waist height	window … sill:900
開き勝手 hand of a door	The hinge side and the direction of swing	hinge: and swing:
引き戸 sliding door	A door that slides sideways	style:sliding
自動ドア automatic door	A door that opens by itself	style:auto
防火戸 fire door	A door that closes to contain a fire	Written as fire: on the asset, and carried

Reading the numbers

Take the output of site.

Site /site (敷地)
  Site area: declared 126.24 m2 / derived 126.24 m2
  Road: /out/road (南側道路) width 6000mm / frontage 10280mm
  Building footprint (horizontal projection, rough): 53.00 m2 → building coverage ratio 42.0%
  Total floor area: 92.75 m2 → floor area ratio 73.5%

• declared is the survey figure the writer put in area:; derived is what came out of the composition. If they disagree, the finding site.area says so.
• width is the width written in road:; frontage is the derived length of adjacency.
• The word rough is a disclaimer: the treatment of canopies and balconies does not reach the resolution of practice.

All of this is judgement, not guarantee. koyu checks nothing against a permitted limit and applies no relaxation.

Beyond this page

• The words of koyu — glossary
• The list of validation rules — validation
• The site question in detail — koyu site