Understanding how to calculate FAR (Floor Area Ratio) and OSR (Open Space Ratio) is the most critical technical step before designing any building in Thailand. These two legal requirements from the urban planning law dictate the maximum size of your project and the minimum "breathing space" you must provide. This guide breaks down the simple formulas, shows which areas count (and which don't), and provides clear examples to help you plan your development correctly from day one.

1. What is FAR? Understanding the Construction "Quota" Before Design

When planning to build, the first question that arises is, "How big of a building can I construct on my land?" The answer depends not only on the size of the land but also on urban planning regulations known as FAR or Floor Area Ratio.

Simply put, FAR is the "quota" or "right" to build floor area that the urban planning law assigns to each plot of land. It is the ratio between the "total building floor area" and the "total land area."

1.1 Calculation Formula and Clear Examples

In practice, we typically use the FAR value specified by law to calculate the maximum building area we can construct.

Formula: Maximum Total Building Area (sq m..) = Land Area (sq m..) x FAR Value

Example: You have a plot of land that is 100 square feet (equivalent to 400 square meters), and upon checking, you find your land is in a zone where the urban plan specifies an FAR of 6:1.

• Maximum total building area you can build = 400 sq.m. x 6 = 2,400 square meters

This means that no matter how many floors you design your building to have, the total usable area of all floors must not exceed 2,400 square meters. For instance:

• You could build a 4-story building with 60s.q s. . .m. per floor.
• Or build an 8-story building with 300 sq. . per floor.

The design must also comply with other regulations, such as building height and setbacks.

1.2 Key Question: Which Areas are "Counted" and "Not Counted" in FAR?

This is a very important point, as not all areas of a building are included in the FAR calculation. According to the Bangkok Metropolitan Administration ordinances (which are often used as a standard reference), it can be broken down as follows:

Areas "Counted" in FAR:

• All interior usable spaces, such as bedrooms, offices, kitchens, and living rooms.
• Common areas like hallways, lobbies, and public restrooms.

Areas "Often Exempted" (Not Counted in FAR):

• Parking spaces and driveways are within the building.
• Covered balconies (mostly exempt, but with conditions on size).
• Elevator shafts, stairwells, and fire escapes.
• Control rooms, mechanical rooms, and rooms for building systems.
• Rooftop areas without a permanent roof cover.

Note: Exemptions may vary in detail by locality. You should always check the ordinances for that specific area.

1.3 How to Find the FAR for Your Land?

The FAR value is not the same everywhere but is specified in the "Ministerial Regulation on the Comprehensive City Plan" for each province or area, varying according to the "color zone."

• Check the color zone: Find out which color zone your land is located in (e.g., Yellow Y.3, Orange Y.6, Red P.4).
• Read the regulations: The text of that comprehensive plan regulation will have a table or statement specifying the FAR and OSR for each color-coded land type.

1.4 Why is FAR Important? (4 Key Points)

1. Controls Urban Density: It is the primary tool to prevent the construction of buildings that are too dense for the capacity of local public utilities, such as electricity, water, and traffic.
2. Determines Land Value and Potential: Land in the same location but with a higher FAR has greater potential for larger projects and thus a higher value.
3. The First Design Brief: Architects and developers use the FAR value as the starting point to determine the size, number of floors, and overall feasibility of a project.
4. Promotes Development Aligned with City Policy: The government may use "FAR Bonuses" to incentivize projects that provide public benefits, such as creating green spaces, constructing energy-efficient buildings, or developing projects near mass transit lines.

Therefore, understanding FAR is not just about numbers; it's about understanding the "maximum potential" of the land you own and is the most crucial first step in developing a project correctly and cost-effectively.

2. What is OSR? The "Breathing Space" is Legally Required in Projects

After knowing the "quota" for building area from FAR, there is another equally important regulation in the urban planning law: OSR, or Open Space Ratio. This regulation dictates "what proportion of your project must be open, uncovered space."

To put it visually, OSR is about setting aside a portion of the project as "breathing space" for planting trees, receiving sunlight, ventilation, and water absorption, which is essential for developing livable and sustainable cities.

2.1 Calculation Formula and Clear Examples

OSR is the ratio of "open space" to the "total building area" (which we calculate from FAR).

Formula: Minimum Required Open Space (sq m.) = Total Building Area (sq m.) x OSR Value (%)

Example: Your project has a total building area (from FAR) of 2,000 square meters, and your land is in a zone where the urban plan specifies an OSR of 7.5%.

• Minimum required open space for this project = 2,000 sq.m. x 7.5% (or 0.075) = 150 square meters

Therefore, in your design, you must allocate a total of at least 150 square meters of open, unroofed space.

2.2 Key Question: What Kind of Areas Count as "Open Space" for OSR?

This is a point that must be clearly understood to avoid design errors.

Areas that "Count" as OSR:

• Green Areas: Such as lawns, gardens, shrubs, and trees.
• Water Features: Such as outdoor swimming pools, ponds, and fountains.
• Multi-purpose Plazas: Such as plazas paved with interlocking blocks, or hardscaped areas for walkways and outdoor activities.
• Outdoor Parking: Parking areas that are not covered by a roof.

Areas that do NOT "Count" as OSR:

• Any area underneath the building.
• Any area covered by a permanent roof, such as a built-in carport or a covered balcony.
• The rooftop area of the building.

2.3 How to Find the OSR for Your Land?

Like FAR, the OSR value is specified in the "Ministerial Regulation on the Comprehensive City Plan" for each province or area and is listed alongside the FAR value for each "color zone."

2.4 Why is OSR Important? (3 Key Points)

1. Promotes Quality of Life and Environment: OSR ensures that every project has sufficient green and open space, reducing congestion and creating a better environment for residents, making the city as a whole more livable.
2. Aids in Water Management and Heat Reduction: Open spaces act as natural areas for rainwater absorption, helping to reduce flooding. Green spaces also help lower the ambient temperature within the project (the Urban Heat Island Effect).
3. Increases Safety and Utility: Open space around a building provides access for emergency vehicles (like fire trucks) and also serves as a multi-purpose area for recreational activities.

3. The Relationship Between FAR and OSR: Balancing "Built Area" and "Open Space"

FAR and OSR are like two sides of the same coin in urban planning law; they cannot be separated. Both values work together to control the physical characteristics of a city and originate from the same source: the "Ministerial Regulation on the Comprehensive City Plan" for each area.

• FAR tells you: The maximum "quota to build" floor area you have.
• OSR tells you: From that built quota, the minimum "proportion of open space" you must have.

"Color Zones" Determine the Fate of FAR and OSR

FAR and OSR values are set differently in each color-zoned area to ensure development aligns with city policy. The relationship is clear:

• Low-Density Residential Areas (e.g., Yellow):
  
  • Tend to have a low FAR (e.g., 2:1) to limit building size.
  • Paired with a high OSR (e.g., 10%) to mandate large gardens and open spaces.
• Central Commercial Areas (e.g., Red):
  
  • Tend to have a very high FAR (e.g., 8:1 or 10:1) to promote the construction of tall, efficient buildings.
  • Paired with a low OSR (e.g., 5%), but still requiring a minimum amount of open space.

A Real-World Example: The Balancing Act of Design

Suppose an architect is tasked with designing a condominium on a 1-rai (1,600 square meters) plot of land located in an Orange Zone (Y.7), with the following regulations:

• FAR = 6:1
• OSR = 5%

The architect begins by calculating the project's feasibility:

Step 1 (Calculate Maximum Built Area from FAR):

• Land Area 1,600 sq.m. x FAR Value 6 = 9,600 square meters
• Conclusion: The architect has a "quota" to design a total building area (including saleable and common areas) of no more than 9,600 sq m.

Step 2 (Calculate Minimum Open Space from OSR):

• Total Building Area 9,600 sq.m. x OSR Value 5% = 480 square meters
• Conclusion: On this 1,600 sq m. plot, there must be at least 480 sq m. of open, unroofed space (e.g., gardens, outdoor pool, internal roads).

The Design Challenge: The architect's task is to design a building with 9,600 sq m. of usable space on a 1,600 sq m. plot, while ensuring that the building footprint and other elements leave at least 480 sq m. of open ground space, and still comply with laws on setbacks and parking. This is the equation that requires a perfect balance.

Summary of Impacts on Design and Development

• Defines Project Size and Scope: FAR sets the "ceiling" for the project's size, while OSR sets the "floor" for its green and open space.
• Dictates Layout and Building Massing: The architect must choose whether to design a tall building with plenty of surrounding space or a low-rise building that spreads out (as long as it doesn't violate FAR/OSR).
• Creates a Complex Puzzle with Other Regulations: The design must always consider other variables simultaneously, including setbacks from roads and neighbors, the legally required number of parking spaces, and access for fire trucks, all of which use up ground-level space.

4. Supplementary Requirements to Consider: Parking, Open Space, and Water Management

In addition to the main regulations of FAR and OSR, there are three other important supplementary regulations, especially in urban areas, that directly impact the layout and budget of a project: parking, multi-purpose open space, and rainwater management.

4.1 Parking Requirements

The law requires certain types of buildings to provide adequate parking in proportion to their use to reduce traffic problems and their impact on public areas.

• Source of Law: Ministerial Regulation No. 7 (B.E. 2549) issued under the Building Control Act, and local ordinances (e.g., Bangkok Metropolitan Administration Ordinance) may provide additional details.
• Applicable Buildings: Mostly buildings with a large number of users, such as:
  
  • Condominiums
  • Hotels
  • Office buildings
  • Department stores, shopping centers
  • Hospitals, medical facilities
• Calculation Example (per BMA Ordinance):
  
  • Office Building: 1 parking space for every 60 square meters of area.
  • Condominium: 1 parking space for every 1 unit (for units 60 sq m. or larger) or every 2 units (for units smaller than 60 sq.m.).
• Impact on Design: The required number of parking spaces directly affects the building design. It may be necessary to build a basement or a separate parking structure, which impacts the budget and other open areas of the project.

4.2 Multi-Purpose Open Space per Specific Requirements (in addition to OSR)

For large-scale projects or land development projects, the open space required by OSR may not be sufficient. The law often requires additional open space for specific purposes.

• Source of Law: Often a requirement in the Environmental Impact Assessment (EIA) Report for large projects or under the Land Development Act for housing estates.
• Objective: To enhance the quality of life for residents and reduce the impact on the surrounding community.
• Examples:
  
  • Playground
  • Recreational areas or a public park for the project.
  • The Buffer Zone is a green belt between the project and adjacent communities.
• Impact on Design: A portion of the land must be specifically allocated for these activities, which affects the overall layout of the project.

4.3 Rainwater Management and Retention Ponds

To help solve flooding problems in urban areas, new construction projects, especially in Bangkok, are often required to have rainwater management systems.

• Source of Law: A strict requirement under local ordinances, such as the Bangkok Metropolitan Administration Ordinance.
• Principle: To create a "water retention area," also known as a "monkey cheek," within the project to temporarily hold rainwater that falls on the property and then slowly release it into the public drainage system later. This prevents overwhelming the system and causing flash floods.
• Forms: Can be either an above-ground retention pond designed as part of a garden or a large underground storage tank.
• Impact on Design: This is a major consideration for landscape architecture and civil engineering. The pond must be calculated to be of sufficient size and placed in a suitable location, which inevitably uses up a part of the project's area.

5. Scenario Example: The Land Analysis of "Khun Ann" and "Khun Boy"

• Khun Ann: An investor who dreams of building a small, charming "boutique hotel" with a beautiful garden.
• Khun Boy: A real estate developer who wants to build a medium-sized "condominium" near a mass transit station.

Both are interested in buying a plot of land of the same size, 1 rai (1,600 square meters), but in different locations. Before deciding to purchase, they hire an architect to conduct a "Project Feasibility Study" using FAR and OSR as key indicators.

Khun Ann's Project: The Boutique Hotel on a Sukhumvit Soi

• Location: A 1-rai plot on a sub-soil in the Sukhumvit area.
• Urban Plan Check: The architect finds the land is in an Orange Zone (Y.6 - Medium-Density Residential).
• Regulations Found:
  
  • FAR = 4. : 1
  • OSR = 6.5 %

The architect begins calculating the potential of this land:

1. Finding the maximum construction "quota" (from FAR):
   
   • Land Area 1,600 sq.m. x FAR 4.5 = 7,200 square meters
   • Architect's summary: "On this plot, we have a quota to build a total building area (including rooms, lobby, hallways) of no more than 7,200 sq.m."
2. Finding the minimum required "open space" (from OSR):
   
   • Building Area 7,200 sq.m. x OSR 6.5% = 468 square meters
   • Architect's summary: "And in the project, we must have at least 468 sq m. of open, unroofed space, such as a garden, an outdoor pool, or walkways around the building."

Analysis: The figure of 7,200 sq m. is suitable for a small boutique hotel project, and the requirement for 468 sq m. of open space aligns with Khun Ann's desire for a beautiful garden and relaxation area. Conclusion: This plot of land has a "high feasibility" for Khun Ann's project.

Khun Boy's Project: The Condominium Near the BTS

• Location: A 1-rai plot on a main road, near a BTS station.
• Urban Plan Check: The architect finds the land is in a Brown Zone (Y.9 - High-Density Residential).
• Regulations Found:
  
  • FAR = 7:
  • OSR = 4 %

The architect begins calculating the potential of this land:

1. Finding the maximum construction "quota" (from FAR):
   
   • Land Area 1,600 sq.m. x FAR 7 = 11,200 square meters
   • Architect's summary: "This land has very high potential. We have a quota to build up to 11,200 sq.m. of floor area."
2. Finding the minimum required "open space" (from OSR):
   
   • Building Area 11,200 sq.m. x OSR 4% = 448 square meters
   • Architect's summary: "Even though it's a high-density area, we still need at least 448 sq m. of exterior open space."

Analysis: With the potential to build up to 11,200 sq.m, this land can be developed into a medium-sized condominium project with enough units to be a worthwhile investment. Conclusion: This plot of land has a "high feasibility" for Khun Boy's project.

Conclusion from the Scenarios:

It is clear that land of the same size (1 rai), when located in "different color zones," has vastly different development potential (7,200 sq m. vs. 11,200 sq.m.). Understanding and calculating FAR and OSR from the very first step is, therefore, the key to helping investors and project owners assess feasibility, plan their business, and make accurate decisions before making a massive investment.

6. Conclusion

In summary, FAR and OSR are not merely technical numbers, but are key tools of the Urban Planning Law that work in tandem to define the physical 'blueprint' of a city. Correctly understanding and calculating these two values from the beginning is, therefore, the most critical step in assessing the true potential of a piece of land. Therefore, expertly analyzing these numbers is not just about calculating area, but about transforming legal constraints into a clear strategic plan to create a project that is balanced, valuable, and truly sustainable.

References

• The Urban Planning Act, B.E. 2562 (พระราชบัญญัติการผังเมือง พ.ศ. 2562)
• Ministerial Regulations on the Comprehensive City Plan (e.g., for Bangkok) (กฎกระทรวงให้ใช้บังคับผังเมืองรวม)
• Building Control Act, B.E. 2522 (พระราชบัญญัติควบคุมอาคาร พ.ศ. 2522)
• Ministerial Regulation No. 7 (B.E. 2549) (กฎกระทรวง ฉบับที่ 7 (พ.ศ. 2549))
• The Land Development Act (พระราชบัญญัติการจัดสรรที่ดิน)
• Enhancement and Conservation of National Environmental Quality Act (regarding EIA) (พระราชบัญญัติส่งเสริมและรักษาคุณภาพสิ่งแวดล้อมแห่งชาติ)
• Relevant Local Ordinances, especially the Bangkok Metropolitan Administration Ordinance (ข้อบัญญัติท้องถิ่น โดยเฉพาะข้อบัญญัติกรุงเทพมหานคร)