Building Code
Road Slant Restriction
Road slant restriction, or 道路斜線制限（どうろしゃせんせいげん）, controls building height from the street side to protect light, air, and openness along roads.
- Japanese Term
- 道路斜線制限
- （どうろしゃせんせいげん）
Overview
Editorial ExplanationRoad slant restriction, 道路斜線制限（どうろしゃせんせいげん）, controls street-side building height and envelope.
Road slant restriction, 道路斜線制限（どうろしゃせんせいげん）, is a Japanese height-envelope rule that controls building height from the front road side. Its purpose is to keep the space above streets open enough for daylight, ventilation, sky visibility, and a reasonable pedestrian scale. It is not a FAR rule about total floor area; it is an envelope rule that shapes street-facing facades, upper setbacks, and roof profiles.
It keeps the space above streets open enough for daylight, ventilation, sky visibility, and a reasonable pedestrian scale.
Exam Snapshot
Exam Reference- 1Common exam point: read Building Standards Act Article 56 paragraph 1 item 1 together with Appended Table 3. First locate the opposite road boundary line.
- 2Fill-in style keyword: horizontal distance. The core reading is “horizontal distance from the relevant part to the opposite road boundary line × slope ratio.”
- 3Numeric exam point: residential zoning is commonly 1.25; commercial and industrial categories are commonly 1.5. Do not memorize numbers without checking zoning.
- 4Setback relaxation: questions may ask for setback distance and outward direction. In study terms, the opposite road boundary line is treated as shifted outward by the setback distance.
Required Terms
Core Diagram
Editorial ExplanationCore Concept
Height H is read from horizontal distance D and the applicable slope measured from the opposite road boundary.
Exam Solving
Residential study example: D = 4 m + 2 m, so H = 6 m × 1.25 = 7.5 m.
Common Mistake
Do not start the road slant line only from the site-side boundary or from setback distance.
Future Diagram
Key Formula
Editorial ExplanationRoad Slant Restriction
Height limit = horizontal distance × slope ratio
Learning formula only. Final application requires zoning, road conditions, applicable distance, Appended Table 3, and current legal text.
- Horizontal distance
- Distance from the opposite road boundary line to the relevant building part.
- Slope ratio
- Common study values are 1.25 for residential and 1.5 for other zones; legal confirmation is required.
- Starting line
- Measured from the opposite road boundary line, not from an arbitrary wall line.
Variables & Terms
Editorial ExplanationVariables
- Horizontal distance
- Distance from the opposite road boundary line to the relevant building part.
- Slope ratio
- Common study values are 1.25 for residential and 1.5 for other zones; legal confirmation is required.
- Starting line
- Measured from the opposite road boundary line, not from an arbitrary wall line.
Terms
- Study formula: road slant height limit = horizontal distance from the opposite road boundary line × slope ratio. This is a learning formula; final application requires checking zoning, road conditions, Appended Table 3, and current legal text.
- The opposite road boundary line, 前面道路の反対側境界線, means the boundary line on the far side of the front road. The slant is not casually measured from the building wall.
- Horizontal distance means the plan distance from that starting line to the relevant part of the building. The farther the building part is from the road, the more height is usually allowed.
- Road width matters because it contributes to the horizontal distance. A building along a 4m road has a tighter street-side envelope than one along a wider road, all else being equal.
- Study slope table: residential zoning is commonly studied as 1.25; other zones are commonly studied as 1.5. The exact value must be checked by zoning category, FAR condition, applicable distance, and Appended Table 3.
- Applicable distance means the range within which the road slant rule applies. The slant is not simply extended forever without limit.
- Setback relaxation / 後退緩和: when a building is set back from the road boundary, the horizontal distance increases, so the allowed height can become less restrictive. This is why setback affects height, not only plan layout.
- Sky factor, 天空率（てんくうりつ）, is an alternative verification route. It does not erase the rule; it compares how much sky is visible from the street-side reference condition.
- Comparison: road slant restriction controls height and street-facing massing; FAR controls total floor area.
- Comparison: road slant protects street-side openness; north-side slant protects the northern neighboring residential environment.
Applicability Check
Editorial Explanation- Step 1: identify front road width, the site boundary, and the opposite road boundary line.
- Step 2: identify the zoning category and the study slope ratio. Residential is often studied as 1.25; other zones as 1.5; legal confirmation is required.
- Step 3: measure the horizontal distance from the opposite road boundary line to each relevant building part and apply: height limit = horizontal distance × slope ratio.
- Step 4: check applicable distance. Road slant is not an infinite envelope with no table conditions.
- Step 5: if the building is set back from the road, recalculate the horizontal distance. A 2m setback can make the allowed height higher in the study model.
- Diagram Notes: a future diagram should label the road, opposite road boundary line, site-side road boundary, building mass, horizontal distance, slant line, buildable envelope, and any volume above the line.
- Design meaning: the rule affects facade setback, upper-floor trimming, roof geometry, penthouse placement, and the street section.
Calculation Process
Editorial Explanation- Step 1
Identify road and starting line
Confirm front road width, site boundary, and the opposite road boundary line.
- Step 2
Check slope and applicable distance
Identify zoning, study slope ratio, and whether the building part is within the applicable distance.
- Step 3
Apply the formula
Measure horizontal distance and apply: height limit = horizontal distance × slope ratio.
- Step 4
Recheck setback
If the building is set back from the road, recalculate horizontal distance; setback can make the envelope less restrictive.
Worked Examples
ExampleBasic Example
Problem
4m road, residential zoning, near road boundary
- Step 1Horizontal distance is about 4m.
- Step 2Height limit = 4m × 1.25 = 5m.
Answer
The exam point is the starting line: measure from the opposite road boundary line, not from an arbitrary wall line.
Exam Example
Problem
4m road, residential zoning, 2m setback
- Step 1Horizontal distance is about 4m + 2m = 6m.
- Step 2Height limit = 6m × 1.25 = 7.5m.
Answer
For setback relaxation, read the opposite road boundary as being shifted outward by the setback distance in the study model.
Slope Check
Problem
Same distance, other-zone slope
- Step 1If horizontal distance is 6m and the slope is 1.5, height limit = 6m × 1.5 = 9m.
Answer
Exam questions often test residential 1.25 versus commercial/industrial 1.5 through zoning and Appended Table 3.
Condition Check
Problem
Two front roads or open space across the road
- Step 1With two or more front roads, questions may use the widest road for relaxation logic.
- Step 2If a park, plaza, or water surface is across the road, the assumed opposite boundary line may shift.
Answer
Mark conditions before calculating.
Examples
Example- Example 1: front road 4m, residential zoning, building near the road boundary. The study horizontal distance from the opposite road boundary to the site-side road boundary is about 4m. Height limit = 4m × 1.25 = 5m. The street-side upper volume may need to step back or be cut.
- Example 2: same 4m road and residential zoning, but the building is set back 2m. Horizontal distance is about 4m + 2m = 6m. Height limit = 6m × 1.25 = 7.5m. Setback makes the envelope less restrictive, but not unlimited.
- Example 3: if the horizontal distance is 6m and the study slope is 1.5 for a non-residential condition, height limit = 6m × 1.5 = 9m. Zoning can change the street envelope.
- Sky-factor example: when a proposal exceeds the ordinary slant plane, 天空率（てんくうりつ） does not mean “no limit”; it means the sky visibility must be compared through an alternative method.
Comparison Table
Editorial ExplanationRoad Slant Restriction vs FAR
| Road Slant Restriction | FAR | |
|---|---|---|
| Controls | Street-side height and massing | Total floor area |
| Focus | Horizontal distance, slope, street openness | Total floor area, site area, designated FAR |
| Trap | FAR compliance does not guarantee envelope compliance | FAR is not a street-side height rule |
Road Slant vs North-side Slant
| Road Slant | North-side Slant | |
|---|---|---|
| Starting side | Road side | North-side neighboring boundary |
| Purpose | Street openness | Northern neighboring residential environment |
| Design effect | Facade setback, upper trimming, street section | Roof shape, north-side setback, residential massing |
Exam Preparation
Exam Reference- Common exam point: read Building Standards Act Article 56 paragraph 1 item 1 together with Appended Table 3. First locate the opposite road boundary line.
- Fill-in style keyword: horizontal distance. The core reading is “horizontal distance from the relevant part to the opposite road boundary line × slope ratio.”
- Numeric exam point: residential zoning is commonly 1.25; commercial and industrial categories are commonly 1.5. Do not memorize numbers without checking zoning.
- Setback relaxation: questions may ask for setback distance and outward direction. In study terms, the opposite road boundary line is treated as shifted outward by the setback distance.
- Two or more front roads: watch for the widest front road as an exam keyword before applying relaxation logic.
- Park, plaza, or water surface across the road: the opposite boundary line may be read at the far side of that open space. Do not freeze the original road edge.
- Problem-solving order: draw the road and opposite boundary → write horizontal distance → check zoning slope → check setback, multiple-road, and park/water conditions → calculate height.
Common Mistakes
Editorial Explanation- Treating road slant as a FAR problem and assuming total floor area compliance is enough.
- Forgetting that the starting line is the opposite road boundary line.
- Memorizing only 1.25 or 1.5 without checking zoning, applicable distance, and Appended Table 3.
- Thinking setback is only a plan issue and does not affect height calculation.
- Reading sky factor as an unlimited exemption.
- Checking only the maximum height instead of the height of each relevant building part against the slant plane.
Memory Tips
Exam Reference- Road slant order: opposite road boundary line → horizontal distance → slope ratio.
- Residential often means 1.25; commercial and industrial often mean 1.5. Always return to zoning and Appended Table 3.
- Setback is not only a plan issue; it changes the horizontal distance used for road slant.
- For multiple roads, watch for the widest road. For parks, plazas, or water across the road, watch where the boundary is deemed to be.
- 天空率（てんくうりつ） is an alternative verification keyword, not an unlimited exemption.
Next Topic
Related Glossary
Related Code Topics
Further Reading
- Building Standards Act Article 56 paragraph 1 item 1
- Appended Table 3
- e-Gov Building Standards Act
- MLIT group regulation materials
Official Source
道路斜線制限
建築基準法 第56条第1項第1号（要最終確認）
Official Source
道路斜線制限
建築基準法 第56条第1項第1号（要最終確認）
- Regulation
- 建築基準法
- Article Number
- 建築基準法 第56条第1項第1号（要最終確認）
- Note
- 道路斜線制限の根拠条文として扱う学習用参照。別表第3を含めた適用条件は最終確認が必要。
- Verification State
- Draft
- Last Reviewed
- Not Reviewed
道路斜線制限
Guide reference
Government Guide
道路斜線制限
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。道路斜線制限と天空率検討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
別表第3
別表第3（要最終確認）
Official Source
別表第3
別表第3（要最終確認）
- Regulation
- 建築基準法
- Article Number
- 別表第3（要最終確認）
- Note
- 道路斜線制限の適用距離や勾配を読むための学習用参照。表の読み方と用途地域別条件は最終確認が必要。
- 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.