Building Code
Floor Area Ratio
Floor Area Ratio controls the total floor area allowed on a site. In Japan, it must be read together with designated FAR and road-width-based restrictions.
- Japanese Term
- 容積率
- （ようせきりつ）
Overview
Editorial ExplanationFloor Area Ratio, 容積率（ようせきりつ）, is the ratio of 延べ面積 to 敷地面積 and controls overall development intensity.
Floor Area Ratio corresponds to the Japanese term 容積率. It expresses the relationship between total floor area, or 延べ面積, and site area, or 敷地面積. It is a density control for total building volume, not a simple permission to build a certain number of floors. The designated FAR shown in planning information is only one part of the check; narrow frontage roads can impose a stricter limit.
It manages urban density, infrastructure load, and block scale. It does not simply decide how many floors a building may have.
Exam Snapshot
Exam Reference- 1Write down two values: designated FAR and road-width-based FAR.
- 2When a narrow front road appears in the problem, immediately check whether the coefficient rule applies.
- 3Do not mix the residential and non-residential coefficients; infer the likely coefficient from the zoning condition.
- 4After calculation, usually use the stricter limit before converting to floor area.
Required Terms
Core Diagram
Editorial ExplanationCore Concept
FAR controls total building volume, not the ground footprint.
Exam Solving
150 m2 site, designated 300%, 4 m residential road: usable FAR 160%, max total floor area 240 m2.
Common Mistake
The front road width limit may reduce the usable FAR.
Future Diagram
Key Formula
Editorial ExplanationFloor Area Ratio
FAR = 延べ面積 ÷ 敷地面積 × 100%
Educational formula only. Real projects require confirmation of floor-area treatment and local conditions.
- 延べ面積（のべめんせき）
- Total floor area across floors. What counts must be checked under Japanese rules.
- 敷地面積（しきちめんせき）
- The site area used for the calculation.
- Designated FAR
- The FAR limit set by zoning and planning controls.
Usable FAR
Usable FAR = min(designated FAR, road-width-based limit)
Road-width limitation is a key study point. Confirm zoning, road conditions, and Article 52 for formal application.
- Front road width
- The width of the road fronting the site. Narrower roads may lower usable FAR.
- Road-width-based limit
- A study calculation using road width multiplied by the zoning-related coefficient.
- Smaller value
- Exam problems commonly require using the stricter of designated FAR and road-width limit.
Variables & Terms
Editorial ExplanationVariables
- 延べ面積（のべめんせき）
- Total floor area across floors. What counts must be checked under Japanese rules.
- 敷地面積（しきちめんせき）
- The site area used for the calculation.
- Designated FAR
- The FAR limit set by zoning and planning controls.
- Front road width
- The width of the road fronting the site. Narrower roads may lower usable FAR.
- Road-width-based limit
- A study calculation using road width multiplied by the zoning-related coefficient.
- Smaller value
- Exam problems commonly require using the stricter of designated FAR and road-width limit.
Terms
- The basic relationship is FAR = total floor area / site area. The precise legal treatment of floor area must be checked under Japanese rules.
- Designated FAR is set through planning controls such as zoning, but the usable FAR may be reduced by the width of the front road.
- For study purposes, residential zoning commonly uses a road-width coefficient of 4/10, while non-residential zoning commonly uses 6/10. The final applicable value must be checked against the actual law and local conditions.
- FAR does not answer “how many floors can I build?” by itself. Coverage ratio, height limits, slant-plane restrictions, fire rules, structure, and planning constraints also shape the building.
- FAR controls total floor area. Building coverage ratio controls ground coverage. They answer different questions.
Applicability Check
Editorial Explanation- First identify the designated FAR for the site.
- Then check the width of the front road. If the road is narrow, calculate the road-width-based FAR limit.
- As a learning rule of thumb: residential zoning often uses road width × 4/10, while non-residential zoning often uses road width × 6/10. Compare this with the designated FAR and focus on the stricter limit.
- Example: if designated FAR is 200% and the front road is 4m wide, a residential 4/10 coefficient gives 4 × 4/10 = 160%. In a study problem, 160% becomes the limit to watch.
- For real projects, road type, access conditions, local ordinances, relaxations, and municipal interpretations must be checked separately.
Calculation Process
Editorial Explanation- Step 1
Confirm site area
Identify the 敷地面積 given in the problem or project data.
- Step 2
Confirm designated FAR
Read the designated FAR from zoning or planning controls.
- Step 3
Confirm front road width
Check the width of the front road, especially when the road is narrow.
- Step 4
Calculate road-width-based FAR
Use the study coefficient to calculate the road-width-based limit.
- Step 5
Use the smaller value
Compare designated FAR and road-width limit, then use the smaller value as usable FAR.
- Step 6
Calculate maximum total floor area
Maximum 延べ面積 = 敷地面積 × usable FAR.
Worked Examples
ExampleBasic Example
Problem
200㎡ site, designated FAR 200%
- Step 1Maximum 延べ面積 = 200㎡ × 200% = 400㎡.
Answer
This is the basic conversion when only designated FAR is considered.
Exam Example
Problem
150㎡ site, 300% FAR, 4m road, residential zoning
- Step 1Road-width limit = 4m × 4/10 = 160%.
- Step 2Usable FAR = min(300%, 160%) = 160%.
Answer
Maximum 延べ面積 = 150㎡ × 160% = 240㎡.
Examples
Example- On a 100 square meter site with an effective FAR of 160%, the rough study-level total floor area is about 160 square meters.
- A site may show a designated FAR of 300%, but if the front road calculation gives 160%, the project cannot simply use 300% for study purposes.
- A three-story building with a 50 square meter footprint may produce about 150 square meters of floor area, but coverage, slant restrictions, fire rules, and height limits still need separate checks.
Comparison Table
Editorial ExplanationRoad-width Coefficient Study Table
| Zoning Group | Study Calculation | |
|---|---|---|
| Residential zoning | Front road width × 4/10 | Learning simplification; final application depends on law and zoning. |
| Other zoning | Front road width × 6/10 | Learning simplification; final application depends on law and zoning. |
Floor Area Ratio vs Building Coverage Ratio
| Floor Area Ratio | Building Coverage Ratio | |
|---|---|---|
| Controls | Overall development intensity and total floor area | Ground-level site coverage |
| Numerator | 延べ面積 | 建築面積 |
| Design effect | Total volume, floor-count possibility, density | Footprint, open space, fire separation |
| Common misunderstanding | Mistaken as a direct floor-count rule | Mistaken as total floor area control |
Exam Preparation
Exam Reference- Write down two values: designated FAR and road-width-based FAR.
- When a narrow front road appears in the problem, immediately check whether the coefficient rule applies.
- Do not mix the residential and non-residential coefficients; infer the likely coefficient from the zoning condition.
- After calculation, usually use the stricter limit before converting to floor area.
- Keep FAR separate from coverage ratio: FAR asks about total floor area, while coverage asks about footprint.
Common Mistakes
Editorial Explanation- Reading only the designated FAR and ignoring the front road width.
- Treating FAR as a direct floor-count rule.
- Confusing FAR with building coverage ratio: FAR is total floor area, coverage is footprint.
- Forgetting to multiply road width by the zoning coefficient in exam-style problems.
- Treating an educational calculation as a legal conclusion.
Memory Tips
Exam Reference- FAR is about total amount.
- Coverage ratio is about ground coverage.
- The narrower the front road, the lower usable FAR may become.
Next Topic
Related Topics
Related Glossary
Related Code Topics
Further Reading
- Building Standards Act Article 52
- e-Gov Building Standards Act
- MLIT building regulation materials
Official Source
容積率
建築基準法 第52条（要最終確認）
Official Source
容積率
建築基準法 第52条（要最終確認）
- Regulation
- 建築基準法
- Article Number
- 建築基準法 第52条（要最終確認）
- Note
- 容積率制限の根拠条文として扱う学習用参照。条文番号と適用条件は最終確認が必要。
- Verification State
- Draft
- Last Reviewed
- Not Reviewed
容積率
Guide reference
Government Guide
容積率
Guide reference
- Regulation
- 国土交通省 建築基準法集団規定資料
- Article Number
- Guide reference
- Note
- 国土交通省の集団規定に関する説明資料。容積率制限を理解するための政府ガイドであり、条文そのものではない。
- Verification State
- Draft
- Last Reviewed
- Not Reviewed
Educational Disclaimer
This content is provided for educational purposes. It is not legal advice and must not be used as the sole basis for permit applications, design approval, or legal determinations. Always consult official sources, local regulations, and qualified professionals.