Height Ratio Front Road Review

Height Ratio and Front-Road Review

Overview

This skill evaluates Taiwan building-height limits commonly referred to in practice as "height ratio" review.

For non-FAR-control contexts, or when Article 166 does not exclude the height limit, the common front-road-width rule is Building Technical Regulations, Design & Construction volume (建築技術規則建築設計施工編) Article 14:

maximum building height = adopted front road width x 1.5 + 6 m

For FAR-control areas, Chapter 9 applies. The practical "1:3.6" / "3.6" height-slope review comes from Article 164:

H <= 3.6 x (Sw + D)
As <= L x Sw / 2

In current practice, most common urban-planned building sites are FAR-control areas. In those cases, Article 166 usually excludes Article 14(1)'s 1.5W + 6m height-limit part and the under-7m-frontage 9 m height cap. Do not apply those as primary height limits in FAR-control reviews. Still use Article 14(1) to determine Sw for Article 164.

Invoke this skill when:

• A project needs a building-height or front-road-width review
• A FAR-control-area Article 164 height-slope or road-shadow-area review is needed
• The site fronts multiple roads, a designated existing lane, a private passage, a planned roundabout, a road-end condition, a greenbelt, a river, or permanent open space
• Floor-area boundary lines vary by floor or mass, requiring conservative control-point selection
• Residential-zone, non-FAR-area, cross-zoning, or local height limits may also control the result
• A permit memo must explain which road width, which article, and which control point were used

Do not stop at a single formula. The legally important questions are which width may be recognized as the front road width, which article applies, and whether a more conservative floor-area-line control point governs the result.

Article Coverage

Required Inputs

Ask for or identify:

• Site boundary, building line, and all roads or access routes touching the site
• Whether the site is in a FAR-control area, because this usually controls whether Article 14 height limits and the under-7m 9 m cap are excluded
• Urban-plan road width, existing-lane designation, and any wall line
• Whether any access is through a private passage or similar passage
• Whether the site fronts two or more roads and each road width
• Whether the site touches or faces permanent open space, greenbelt, river, park, square, lake, ocean, or other legally nonbuildable land
• Whether the site touches a road end
• Land-use zoning, residential-zone status, non-FAR-control status, and cross-zoning
• Proposed building height, stories, and the zone or building portion tested
• Floor-area boundary lines for each floor or mass being tested
• Candidate Article 164 control points, especially the outermost floor-area-line points facing the building line
• Article 164 projection direction, opposite front-road boundary, frontage length L, and any road-shadow-area drawing or area calculation
• Whether the opposite side of the front road has legally confirmed permanent open space for Article 164 shadow-area doubling
• Local zoning, urban-plan, local-government, or special-building restrictions

If building-line or road-width evidence is missing, mark the conclusion as "requires authority confirmation" rather than treating measured pavement width as final.

Review Algorithm

1. Determine the legal nature of every frontage.

   • Article 1 roads include legally announced roads and existing lanes with designated building line.
   • Private passages and similar passages are not roads unless a specific rule treats them as a front road for height review.

2. Confirm the building height being tested.

   • Use Article 1's building-height definition.
   • Flag roof projections, parapets, roof equipment, non-flat roofs, and site ground differences for separate Article 1 review.

3. Decide whether Chapter 9 FAR-control-area rules govern.

   • If the site is in a FAR-control area, Article 160 points to Chapter 9 unless urban-plan law or plan text says otherwise.
   • Article 166 excludes Article 14(1)'s height-limit part, Article 15, Article 23, Article 26, and Article 27 from FAR-control areas.
   • In common current urban-planned projects, assume FAR-control is likely until checked. If confirmed, do not use 1.5W + 6m or the under-7m 9 m cap as primary height limits unless a separate urban-plan, local, or authority requirement says so.
   • In FAR-control areas, run Article 164 and still flag stricter urban-plan or local rules.

4. Apply Article 14 where applicable.

   • If Article 166 excludes the Article 14 height-limit part, do not run H <= 1.5W + 6m as a governing height cap.
   • Still use Article 14(1)'s front-road-width recognition rules to determine Sw for Article 164.
   • Normal frontage: use front road width.
   • Wall line: measure road width to the wall line.
   • Planned roundabout: use the widest road intersecting the roundabout, then check Article 16 if another side also fronts a road.
   • Private passage used as main access to the building line: treat it as the front road, but if wider than the connected road, use the connected road width.
   • Private passage left inside a site touching the building line: apply Article 16(1) to the portion near the building line; apply Article 14(3) to the remaining portion.
   • Road separated by greenbelt or river: combine both road widths, capped at twice the width of the road directly touching the site.

5. Add the under-7m road rule only where Article 14's height-limit part applies.

   • If adopted front road width is under 7 m, the building height within 3.5 m from the road centerline is capped at 9 m.
   • In FAR-control areas, this cap is usually excluded by Article 166 together with Article 14(1)'s height-limit part.

6. Run Article 164 in FAR-control-area reviews as one integrated geometry check.

   • Establish the front road, building line, opposite front-road boundary, projection direction perpendicular to the building line, Sw, and L.
   • Use Sw based on Article 14(1)'s front-road-width recognition rules.
   • Mark the floor-area boundary line for each tested floor or mass.
   • For D, use the conservative control point on the floor-area boundary line: the closest / outermost point facing the building line for the building part being tested.
   • Pair that control point with the corresponding building-part height H.
   • Check height by H <= 3.6(Sw + D).
   • Project each tested building part at the 3.6:1 slope in the direction perpendicular to the building line and calculate the road-shadow area As falling on the front road.
   • Check As <= L x Sw / 2.
   • Confirm the shadow maximum does not exceed the opposite front-road boundary.
   • If the opposite side of the front road has legally confirmed permanent open space, the allowable shadow area may be doubled.
   • If floor-area boundary lines vary by floor, bend, curve, or step back, test all relevant outer points and use the smallest D, largest As, or strictest result.

7. Check Article 15 permanent open space where not excluded by Article 166.

   • Opposite-side permanent open space: height is capped by (road width + open-space depth) x 1.5, and also by widest road width x 2 + 6 m.
   • Site touching surrounding permanent open space: open-space width and depth or depth sum must reach 20 m; height is capped by widest road width x 2 + 6 m.
   • Partial frontage/facing: only the matching extension zone, up to 30 m, benefits from the rule.
   • If Article 14(5) also applies, use the wider applicable result.

8. Check Article 16 multiple-road zoning.

   • Zone from the widest road boundary: depth 2 x Wmax, capped at 30 m, uses the widest road.
   • Outside that zone, within 10 m from other road centerlines, the next-road zone uses depth 2 x Wnext, capped at 30 m; repeat in road-width order.
   • Remaining area outside the first two categories uses the widest road.
   • Compute height separately for each zone.
   • For four-frontage sites, use the B, B1, B2, B3, B4 workflow in the Article 16 method below.

9. Check Article 19 road-end condition.

   • If the site touches a road end, use that road width as the front road.
   • If another side touches a wider road, height is not limited by the road-end width; continue with the wider-road analysis.

10. Check related height restrictions.

• Article 8: if the site touches an existing lane but also touches a wider road and is a corner lot, height is not limited by the existing-lane width.
• Article 23: residential-zone buildings are generally capped at 21 m and 7 stories unless the road-width/open-space exceptions apply; over 36 m also requires Article 24 review.
• Article 24: non-FAR-control areas are generally capped at 36 m and 12 stories unless the listed site, road, and open-space conditions apply.
• Article 27: added stories/height may require added open space where applicable, and cannot exceed Section 3 height limits where applicable.
• Article 29: if the site crosses multiple zoning districts, calculate height separately by district.
• Always flag stricter urban-plan, zoning, local-government, special-use, and special-building rules.

11. Output the conclusion.

• State evidence, adopted width, applicable article, control point, formula, height cap, proposed height, result, and any authority-confirmation item.

Key Formulas

Article 14 base:
Hmax = Wfront x 1.5 + 6 m
Use only where Article 14's height-limit part is not excluded by Article 166.

Article 14 under-7m frontage:
Within 3.5 m from the road centerline, Hmax = 9 m
Usually excluded in FAR-control areas by Article 166.

Article 15 opposite permanent open space:
Hmax <= (road width + permanent open-space depth) x 1.5
Hmax <= widest road width x 2 + 6 m

Article 15 surrounding permanent open space:
If open-space width and depth/depth-sum >= 20 m,
Hmax <= widest road width x 2 + 6 m

Article 164 FAR-control-area height slope:
H <= 3.6 x (Sw + D)
As <= L x Sw / 2

Article 164 shadow-depth quick check:
horizontal projection depth from control point = H / 3.6
road-shadow depth ~= max(0, H / 3.6 - D)

When conservative office-standard review is requested, measure D from the outermost floor-area boundary line control point facing the building line.

Conservative Article 164 Control-Point Method

Use this method when the user says the office reviews from the strictest point, or when floor-area boundary lines vary by point:

1. Extract each floor or mass floor-area boundary line.
2. Find the side facing the relevant building line.
3. Mark every turn point, curve tangent, local protrusion, and closest point to the building line.
4. Measure D from each candidate point toward the building line, in the direction used for Article 164's perpendicular projection to the building line.
5. Pair each control point with that building part's H.
6. Use the smallest D, lowest 3.6(Sw + D), or most unfavorable exceedance as the controlling result.

Do not use average distance, centroid distance, the most recessed point, or the largest setback to increase allowed height.