balcony-lobby-far-recalculation

name	balcony-lobby-far-recalculation
description	This skill should be used when calculating Taiwan FAR gross-floor-area inclusion for balcony and shared stair/elevator lobby areas under Building Technical Regulations Article 162, including the separate 10% balcony cap, separate 10% lobby cap, combined 15% balcony-plus-lobby cap, and avoiding double-counting already-included excess area.
license	CC-BY-SA-4.0
compatibility	claude-code,opencode,agent-skills
metadata	{"audience":"architects","region":"taiwan","class":"C"}

Balcony and Lobby FAR Recalculation

Overview

Use this skill when reviewing Taiwan building-permit FAR calculations for:

• Balcony area (陽臺面積) exemption up to 10% of the floor area
• Shared stair/elevator lobby area (梯廳面積) exemption up to 10% of the floor area
• Combined balcony plus lobby exemption up to 15% of the floor area
• Recalculated area that must be included in FAR gross floor area (回計容積面積)

The key practitioner issue is avoiding double-counting: once balcony or lobby area has already exceeded its own 10% cap and has been included in FAR, the combined 15% check should use only the remaining exempt candidates, not the original raw balcony plus lobby total.

Legal Basis

Building Technical Regulations, Design and Construction, Article 162, Paragraph 1, Subparagraph 1:

• Balcony area within 10% of that floor's area may be excluded from FAR gross floor area.
• Shared stair/elevator lobby area within 10% of that floor's area may be excluded, provided the lobby clear depth is at least 2 m.
• If balcony plus lobby area exceeds 15% of that floor's area, the excess must be included.

Always verify local review-sheet conventions and current official text before final filing.

Inputs

Symbol	Meaning
F	Floor area for the reviewed story
B	Total balcony area for the story
L	Total eligible shared stair/elevator lobby area for the story
Cap10	F * 0.10, rounded to two decimals before use
Cap15	F * 0.15, rounded to two decimals before use

Only include lobby area in L after confirming the lobby meets basic eligibility, especially the 2 m finished clear-depth requirement.

Algorithm

Cap10 = round(F * 0.10, 2)
Cap15 = round(F * 0.15, 2)

balcony_exempt_candidate = min(B, Cap10)
balcony_excess_10 = B - balcony_exempt_candidate

lobby_exempt_candidate = min(L, Cap10)
lobby_excess_10 = L - lobby_exempt_candidate

combined_exempt_candidate = balcony_exempt_candidate + lobby_exempt_candidate
combined_excess_15 = max(combined_exempt_candidate - Cap15, 0)

total_far_included_area =
  balcony_excess_10
  + lobby_excess_10
  + combined_excess_15

final_exempt_area = B + L - total_far_included_area

Review Logic

1. Check balcony against the 10% cap and record balcony excess.
2. Check lobby against the 10% cap and record lobby excess.
3. Cap each item at its 10% exempt candidate before the combined 15% check.
4. Check min(B, Cap10) + min(L, Cap10) against Cap15.
5. Add the two separate 10% excess values and the combined 15% excess value.

Do not calculate separate 10% excess first and then check raw B + L against 15%, because that can include the same excess area twice.

Decimal Handling

Use one decimal convention throughout the calculation sheet:

• Round Cap10 = F * 0.10 to two decimal places before using it as the 10% exemption cap.
• Round Cap15 = F * 0.15 to two decimal places before using it as the combined 15% exemption cap.
• Use the rounded Cap10 to split each item, so exempt area + included excess area = original balcony or lobby area in the review table.
• Display other output fields to two decimal places.

Use the usual half-up engineering-table convention for midpoint cases.

Worked Examples

Example A

F = 752.58
L = 78.69
B = 2.70

Cap10 = round(752.58 * 0.10, 2) = 75.26
Cap15 = round(752.58 * 0.15, 2) = 112.89

lobby_excess_10 = 78.69 - 75.26 = 3.43
balcony_excess_10 = 0

combined_exempt_candidate = 75.26 + 2.70 = 77.96
combined_excess_15 = 0

total_far_included_area = 3.43

With two-decimal rounding, report 3.43 m2.

Example B

F = 447.48
L = 40.37
B = 36.43

Cap10 = round(447.48 * 0.10, 2) = 44.75
Cap15 = round(447.48 * 0.15, 2) = 67.12

lobby_excess_10 = 0
balcony_excess_10 = 0

combined_exempt_candidate = 40.37 + 36.43 = 76.80
combined_excess_15 = 76.80 - 67.12 = 9.68

With two-decimal rounding, report 9.68 m2.

Example C: Both Items Exceed 10%

F = 500.00
L = 62.00
B = 58.00

Cap10 = round(500.00 * 0.10, 2) = 50.00
Cap15 = round(500.00 * 0.15, 2) = 75.00

lobby_excess_10 = 12.00
balcony_excess_10 = 8.00

combined_exempt_candidate = 50.00 + 50.00 = 100.00
combined_excess_15 = 25.00

total_far_included_area = 12.00 + 8.00 + 25.00 = 45.00
final_exempt_area = 62.00 + 58.00 - 45.00 = 75.00

This preserves the 15% final exempt ceiling without double-counting the 20 m2 that was already included by the separate 10% checks.

Validation Cases

Use these cases to verify that each 10% item split satisfies exempt + included = original area after the 10% cap is rounded.

Case	F	Cap10	Item	Original	Exempt	Included	Sum
A lobby over 10%	752.58	75.26	Lobby	78.69	75.26	3.43	78.69
A balcony under 10%	752.58	75.26	Balcony	2.70	2.70	0.00	2.70
B lobby under 10%	447.48	44.75	Lobby	40.37	40.37	0.00	40.37
B balcony under 10%	447.48	44.75	Balcony	36.43	36.43	0.00	36.43
Exactly rounded 10% cap	333.35	33.34	Lobby	33.34	33.34	0.00	33.34
Just above rounded 10% cap	333.35	33.34	Balcony	33.35	33.34	0.01	33.35
Both items over 10%	500.00	50.00	Lobby	62.00	50.00	12.00	62.00
Both items over 10%	500.00	50.00	Balcony	58.00	50.00	8.00	58.00

Output Checklist

• Confirm lobby clear depth and eligibility before including lobby area as an exempt candidate.
• Show Cap10 and Cap15 for each floor.
• Show separate balcony and lobby 10% excess values.
• Show combined 15% excess using the capped exempt candidates.
• State the decimal rule: round Cap10 and Cap15 to two decimals before using them as caps.

Tool Examples

• taiwan-building-code_search_building_code(query: "建築技術規則 第162條 陽臺 梯廳 百分之十五", limit: 5)
• taiwan-building-code_search_building_interpretations(query: "陽臺 梯廳 容積 15%", limit: 5)

References

• Building Technical Regulations, Design and Construction, Article 162: FAR gross-floor-area calculation and exemption items
• Local building-permit FAR calculation sheets and review conventions