--- type: Skill name: height-ratio-front-road-review description: "This skill should be used when reviewing Taiwan building-height limits controlled by front-road width and FAR-area 3.6:1 height-slope rules: determining the legal front road, calculating Article 14 1.5W+6m where applicable, Article 164 H<=3.6(Sw+D) and shadow-area caps, selecting conservative floor-area-line control points, handling private passages, multiple roads, permanent open space, road-end, residential/non-FAR/cross-zoning checks under Articles 1, 8, 14-19, 23, 24, 27, 29, 160, 164, and 166." license: CC-BY-SA-4.0 compatibility: claude-code,opencode,agent-skills metadata: audience: architects region: taiwan class: C --- # 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: ```text 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: ```text 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 | Article | Topic | Use in review | |---|---|---| | Art. 1 | Definitions | Building height, site ground, road, private passage, similar passage, permanent open space, floor area, setback depth | | Art. 8 | Existing lanes | Existing-lane frontage and wider-road corner-lot exception | | Art. 14 | Base height ratio | `H <= 1.5W + 6m` where not excluded; front-road width recognition cases for `Sw` | | Art. 15 | Permanent open space | Height limits when the site touches or faces permanent open space | | Art. 16 | Multiple roads | Depth-zone method for sites fronting two or more roads | | Art. 17 | Deleted | No current substantive rule | | Art. 18 | Deleted | No current substantive rule | | Art. 19 | Road end | Dead-end frontage rule and wider-road exception | | Art. 23 | Residential zone | 21 m / 7-story cap and exceptions | | Art. 24 | Non-FAR-control area | 36 m / 12-story cap and exceptions | | Art. 27 | Added height and open space | Extra open-space rule; still cannot exceed Section 3 height limits where applicable | | Art. 29 | Cross-zoning site | Height must be calculated separately by zoning district | | Art. 160 | FAR-control areas | FAR-control-area design follows Chapter 9 unless urban-plan rules say otherwise | | Art. 164 | 3.6:1 height slope | `H <= 3.6(Sw + D)`, road-shadow-area limit, and opposite-road-boundary cap | | Art. 166 | FAR-control exclusions | Art. 14(1) height-limit part, Arts. 15, 23, 26, 27, etc. do not apply to FAR-control areas | --- ## 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 ```text 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. ``` ```text 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. ``` ```text Article 15 opposite permanent open space: Hmax <= (road width + permanent open-space depth) x 1.5 Hmax <= widest road width x 2 + 6 m ``` ```text Article 15 surrounding permanent open space: If open-space width and depth/depth-sum >= 20 m, Hmax <= widest road width x 2 + 6 m ``` ```text Article 164 FAR-control-area height slope: H <= 3.6 x (Sw + D) As <= L x Sw / 2 ``` ```text 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. --- ## Article 16 Four-Frontage Method Use this method when a site fronts four roads or when the user describes the whole site as `B` and asks how to identify front-road control areas. The plain-language rule is: give the widest road first priority, then treat the remaining area controlled by the other roads as an undecided pool. Do not assign `B3` or `B4` before `B2` is calculated. `B2` first takes from the undecided pool; then `B3` takes from what remains; the final remainder becomes `B4`. 1. Name the whole site `B`. 2. Sort all frontage roads by width: `W1 > W2 > W3 > W4`. 3. From the `W1` road boundary or building line, offset into the site by `min(2W1, 30 m)`. 4. Assign that first area to `B1`. 5. Define the remaining site as `B - B1`. 6. Inside `B - B1`, use the 10 m road-centerline zones of `W2`, `W3`, and `W4` to identify the undecided pool `U` that may be governed by the other roads. 7. Any part of `B - B1` outside `U` returns to `B1` under Article 16's remaining-area rule. 8. Keep `U` undecided. Do not pre-assign it to `B2`, `B3`, and `B4`. 9. For `W2`, offset from the `W2` road boundary or building line by `min(2W2, 30 m)`. The portion of `U` reached by that offset becomes `B2`. This may consume area that visually seems like future `B3` or `B4`, because those areas are not yet decided. 10. Define the remaining undecided pool as `U - B2`. 11. For `W3`, offset from the `W3` road boundary or building line by `min(2W3, 30 m)`. The portion of `U - B2` reached by that offset becomes `B3`; this may cut into the area that would otherwise remain for `B4`. 12. Assign the final remaining undecided area to `B4`. 13. Use each final area with its own front road: `B1 -> W1`, `B2 -> W2`, `B3 -> W3`, `B4 -> W4`. Practical output should show the whole site `B`, road-width order, the undecided pool `U`, areas returned to `B1`, the sequential `min(2W, 30 m)` offset cuts for `B2` and `B3`, and the final `B1` through `B4` boundaries. --- ## Article 164 Road-Shadow-Area Method Treat the road-shadow-area review as part of the same Article 164 geometry, not as a detached checklist. 1. Confirm the front road, building line, opposite road boundary, `Sw`, and `L`. 2. Confirm the projection direction perpendicular to the building line. 3. Mark each tested floor-area boundary line and its conservative control point. 4. Measure `D` from the control point to the building line and pair it with that building part's `H`. 5. Use `H / 3.6 - D` as a quick check for whether the projected shadow enters the front road, but do not use it as a substitute for area calculation. 6. Draw or calculate the 3.6:1 projection onto the front road and compute `As`. 7. Check both limits: - `H <= 3.6(Sw + D)` - `As <= L x Sw / 2` 8. Confirm no shadow extends beyond the opposite front-road boundary. 9. If claiming shadow-area doubling, verify the opposite-side permanent open space under the legal definition before using `As <= L x Sw`. If the height formula passes but `As` fails, or if `As` passes but the projection extends past the opposite road boundary, the Article 164 result is still not clean. --- ## Decision Checklist | Question | Why it matters | |---|---| | Is each frontage a legal road, designated existing lane, private passage, or similar passage? | Article 1 controls the baseline classification. | | Is the site in a FAR-control area? | Article 160 points to Chapter 9 and Article 166 usually excludes Article 14 height limits, including `1.5W + 6m` and the under-7m 9 m cap. | | Is a wall line designated? | Article 14 measures road width to the wall line. | | Does the site face a planned roundabout? | Article 14 uses the widest road intersecting the roundabout. | | Is main access through a private passage? | Article 14 may treat it as the front road, capped by connected road width. | | Is there a greenbelt or river between roadways? | Article 14 allows combined width, capped at twice the directly adjacent road width. | | Does the site front more than one road? | Article 16 creates depth zones. | | Does the site face or touch permanent open space? | Article 15 may change the height limit and applicable area, unless excluded by Article 166. | | Is the adopted road width under 7 m? | Article 14 adds a 9 m cap in the 3.5 m centerline zone only when Article 14's height-limit part is not excluded by Article 166. | | Is the road a dead end? | Article 19 controls unless another side touches a wider road. | | Is the site in a residential zone or non-FAR-control area? | Articles 23 and 24 may impose separate caps. | | Does the site cross zoning districts? | Article 29 requires separate calculation. | | Does Article 164 apply? | Test `H`, `D`, `Sw`, `As`, and the opposite road boundary. | | Which floor-area-line point is most conservative? | Use the closest / outermost point to the building line, not average or maximum setback. | | Is the opposite front-road boundary shown? | Article 164 requires the shadow maximum not to exceed that boundary. | | Was `As` calculated from the projected road shadow? | Passing `H <= 3.6(Sw + D)` alone does not complete Article 164 review. | | Is opposite-side permanent open space legally confirmed? | Article 164 shadow-area doubling depends on this condition. | --- ## Common Pitfalls - Using actual pavement width without checking building-line designation or urban-plan road width. - Treating a private passage or similar passage as a road without an Article 14 basis. - Applying Article 14, Article 15, or Article 23 in a FAR-control area without checking Article 166. - Treating `1.5W + 6m` or the under-7m 9 m cap as the governing height limit in a FAR-control area, instead of switching to Article 164. - Running Article 164 with average setback, largest setback, inner wall line, or centroid distance instead of the outermost floor-area-line control point. - Using one `D` for the whole building when different floors or masses have different floor-area boundary lines and heights. - Applying the widest road to the whole site even though Article 16 requires depth-zone review. - Combining road widths across a greenbelt or river but forgetting the two-times direct-frontage cap. - Forgetting the 9 m cap when the adopted front road width is under 7 m and Article 14 applies. - Treating permanent open space as simple road widening without Article 15's depth, width, partial-application, and cap checks. - Reviewing residential-zone buildings only by `1.5W + 6m` and missing Article 23 where it applies. - Running only `H <= 3.6(Sw + D)` and forgetting the Article 164 road-shadow-area check `As`. - Treating a passing `As` value as sufficient when the projected shadow still exceeds the opposite front-road boundary. - Doubling Article 164 allowable shadow area without legally confirming opposite-side permanent open space. - Ignoring cross-zoning, local urban-plan text, or stricter local rules. --- ## Output Format Use this format for permit memos or design checks: ```text Site / lot: Zoning: FAR-control area: yes / no Article 166 exclusions: Article 14 height limit / under-7m 9 m cap / Article 15 / Article 23 / Article 27 / other: Frontages reviewed: Road-recognition basis: Special conditions: Applicable article(s): Adopted front road width: Height cap: Article 164 control point / D / As / opposite-boundary check, if applicable: Article 164 opposite permanent open-space doubling, if applicable: Proposed building height: Related height restrictions: Result: Open items / authority confirmation needed: ``` For multiple-road sites, add a zone table: | Zone | Road basis | Depth range | Adopted width | Formula | Height cap | Proposed height | Result | |---|---|---|---|---|---|---|---| For Article 164 reviews, add: | Floor / mass | Floor-area-line control point | Sw | D | H | 3.6(Sw+D) | Shadow beyond opposite boundary? | As | LxSw/2 | Opposite open-space doubling | Result | |---|---|---:|---:|---:|---:|---|---:|---:|---|---| --- ## MCP Tool Examples - `taiwan-building-code_search_building_code(query: "建築技術規則 建築物高度 面前道路", limit: 10)` - `taiwan-building-code_search_building_interpretations(query: "面前道路 私設通路 建築物高度", limit: 10)` - `taiwan-building-code_search_building_interpretations(query: "建築技術規則 第14條 面前道路 綠帶 河川", limit: 10)` - `taiwan-building-code_search_building_interpretations(query: "建築技術規則 第16條 臨接兩條以上道路 高度", limit: 10)` - `taiwan-building-code_search_building_code(query: "建築技術規則 第164條 三點六比一", limit: 10)` - `taiwan-building-code_search_building_interpretations(query: "第164條 樓地板面積線 建築線 D", limit: 10)` ## Sources - 建築技術規則建築設計施工編第 1 條 — Definitions for building height, road, private passage, similar passage, permanent open space, and setback depth https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=1 - 建築技術規則建築設計施工編第 8 條 — Existing lanes and wider-road corner-lot exception https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=8 - 建築技術規則建築設計施工編第 14 條 — Base height ratio and front-road-width recognition https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=14 - 建築技術規則建築設計施工編第 15 條 — Permanent-open-space height rules https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=15 - 建築技術規則建築設計施工編第 16 條 — Multiple-road-site depth zones https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=16 - 建築技術規則建築設計施工編第 19 條 — Road-end conditions https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=19 - 建築技術規則建築設計施工編第 23 條 — Residential-zone height rules https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=23 - 建築技術規則建築設計施工編第 24 條 — Non-FAR-control-area height rules https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=24 - 建築技術規則建築設計施工編第 27 條 — Added height, stories, and open space https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=27 - 建築技術規則建築設計施工編第 29 條 — Cross-zoning height calculation https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=29 - 建築技術規則建築設計施工編第 160 條 — FAR-control-area design follows Chapter 9 https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=160 - 建築技術規則建築設計施工編第 164 條 — 3.6:1 height slope and road-shadow-area limits https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=164 - 建築技術規則建築設計施工編第 166 條 — FAR-control-area exclusions from several height rules https://law.moj.gov.tw/LawClass/LawSingle.aspx?pcode=D0070115&flno=166