Check sewage volume and septic design population before changing a building
Converting part of a home into a café, adding guest rooms or extending a shop can change its sewage requirements even when the floor area barely changes.
Homes, offices, refreshment businesses and general restaurants have different statutory rates in Korea.
Preparing a use-by-use calculation before requesting quotations can help identify a capacity problem while the plans can still be adjusted.
This calculator uses South Korean rules to calculate the baseline treatment population N, daily sewage volume Q and flow-weighted BOD before and after a proposed change.
Treatment-area status, protected-area conditions and existing-facility information then support a preliminary facility and capacity review.
The 2026 rule set was checked on November 9, 2026; Notice No. 2025-165 took effect on October 1, 2025.
These are Korean design assumptions and must not be applied as building or wastewater rules for another country.
Four separate outputs and what they mean
Treatment population N
This is the population obtained from the notice’s room or floor-area formula.
It is not necessarily the actual headcount and must be distinguished from the rated population on an installed product.
Daily sewage volume Q
This expresses daily sewage generation in L/day and m³/day.
Use-specific rates determine this flow; it is not the tank’s storage volume.
Mixed BOD
Biochemical oxygen demand is averaged using each sewage stream’s flow.
This baseline concentration is not an effluent test result or a guarantee of installed treatment performance.
Change in daily volume
The after-change Q minus the before-change Q shows the increase or decrease.
Positive increases support sewer-contribution screening, subject to the authority’s approved baseline.
Match homes, legal rooms and floor area to the drawings
The residential variable R means legal rooms, rather than only the room casually described as the living room.
It includes qualifying rooms used for living, working, assembly or recreation and separate spaces capable of functioning as bedrooms.
In housing, separately provided kitchens, dining rooms, dressing rooms, powder rooms and utility rooms are excluded under the notice’s room definition.
Enter different room arrangements in separate rows so each home receives the correct formula.
Calculation area A may differ from exclusive business area
The relevant gross floor area includes the floors used for that purpose and associated common area, excluding accessory parking.
Mechanical rooms, air-conditioning rooms and canopies that generate no sewage are excluded; do not assume every storage space is automatically deductible.
Where a main warehouse or livestock-building use generates no sewage, calculate the sewage-generating ancillary toilets, management office or showers separately.
Food businesses must also include their reported outdoor business area where the annex requires it.
Capacity P is available only for uses whose annex row provides an alternative population formula and where clear evidence establishes that capacity.
Selecting a P-based formula for N does not convert a floor-area sewage rate into a per-person rate.
Unknown treatment-area or existing-capacity information should remain unknown rather than being entered as zero or filled with an estimate.
Common use-specific sewage and population rates
A is the confirmed floor area allocated to the relevant use.
Multiply the sewage rate by A to obtain L/day, and use the separate N formula to obtain the baseline population.
Selected sewage-generation rates, septic population formulas and BOD values from the annex to Korean Notice No. 2025-165| Use | L/m²/day | N formula | BOD mg/L |
|---|
| Office | 15 | 0.075 × A | 100 |
| Retail shop or market | 15 | 0.075 × A | 250 |
| Refreshment business or qualifying café | 35 | 0.175 × A | 100 |
| General restaurant: Korean or snack food | 60 | 0.175 × A | 330 |
| General restaurant: Chinese food | 60 | 0.175 × A | 550 |
| General restaurant: Japanese food, pub or buffet | 60 | 0.175 × A | 200 |
| General restaurant: Western food | 60 | 0.175 × A | 150 |
| On-site food preparation/retail or delivery-only shop | 30 | 0.150 × A | 130 |
| Rural homestay or tourist pension | 35 | 0.140 × A | 140 |
| Ordinary lodging: no cooking facilities | 20 | 0.080 × A | 70 |
| Ordinary lodging: cooking facilities | 20 | 0.080 × A | 140 |
| Private academy, tutoring facility or reading room | 15 | 0.075 × A | 150 |
For shops and markets, meat/fish trading floor area totaling at least 20% of the relevant gross floor area adds 5 L/m²/day and 50 mg/L BOD under the annex.
A delivery-only business with a customer dining area may instead need refreshment-business or general-restaurant treatment.
A café selling general-restaurant menus also requires a classification check.
Showers in sports facilities and laundry facilities in certain medical uses can require separate additions, so the base use row may not describe the whole building.
Residential formulas and mixed-use totals
Detached and multi-household houses, official residences and farmer housing
N per home = 2.0 + (R − 2) × 0.5
Sum the calculated populations across homes.
Daily residential sewage is 200N L/day in the ordinary case and 170N in eup/myeon areas; farmer housing uses 170N regardless of that area selection.
Apartment-type housing
N per home = 2.7 + (R − 2) × 0.5
This category includes apartments, row houses and qualifying multi-unit housing.
A home consisting of one legal room counts as two people.
Daily sewage is 200N L/day, reduced to 170N in eup/myeon areas.
Calculate each use first, then sum the contributions
Total N = ΣNᵢ · Total Q = ΣQᵢ · Mixed BOD = Σ(Qᵢ × BODᵢ) ÷ ΣQᵢ
Use consistent flow units for all rows when calculating the weighted concentration.
There are 1,000 liters in 1 m³, and a zero-flow row contributes no flow weight.
When total Q is zero, mixed BOD is not calculated.
The formulas preserve fractional N; a rounded-up integer is only a quotation reference and must not replace N inside the residential Q formula.
A practical calculation sequence
- Confirm the address’s treatment-area status, combined or separate sewer arrangement and actual public-treatment connection.
Also establish protected-area status and whether flush toilets are installed.
- Enter before-change homes, legal rooms and use-specific areas from the building register and drawings.
Record any previously approved sewage figure separately from a fresh calculation under the current notice.
- Describe the actual after-change uses, including extension area, reported outdoor food-business area and any rural-homestay conditions.
Check that no physical space has been counted twice across rows.
- Check the existing facility’s type and rated population, then enter an authority-confirmed capacity on the same measurement basis as the sewage comparison.
Outside a treatment area, confirm toilet sewage separately when reviewing the septic-tank exception.
- Review N, Q, mixed BOD and the increase, then copy or save the calculation schedule.
Take unresolved questions and drawings to the designer and local sewer authority before confirming construction scope.
Worked examples: a house and café, and an office conversion
One detached home, R=4, plus a 40 m² café
The house contributes N=3 and Q=0.600 m³/day under the ordinary 200 L/person/day residential rate.
A qualifying 40 m² refreshment business contributes N=7 and Q=1.400 m³/day.
The combined result is N=10 and Q=2.000 m³/day.
Mixed BOD is (600×200+1,400×100)÷2,000=130 mg/L.
This does not exceed the ordinary outside-area 2 m³ threshold, while protected-area and existing-capacity checks still remain.
Eup/myeon location or a 40.01 m² café
In an eup/myeon area, the same house contributes 0.510 m³/day and the total becomes 1.910 m³/day.
Mixed BOD is approximately 126.702 mg/L, with the same total N=10.
Using the ordinary residential rate and increasing café area to 40.01 m² gives Q=2.00035 m³/day, which exceeds 2 m³/day.
Do not classify a threshold using only a rounded display value.
Convert a 100 m² office into a Korean-food general restaurant
The office has N=7.5 and Q=1.500 m³/day; the restaurant has N=17.5 and Q=6.000 m³/day.
The increase is 4.500 m³/day, and the baseline BOD changes from 100 to 330 mg/L.
An increase below 10 m³/day does not establish that the existing septic tank is adequate or that every contribution is waived.
A change of use can alter the facility review without any floor-area increase.
Facility screening: the 2 m³ and protected-area 1 m³ rules
Ordinary location outside a sewage treatment area
Enforcement Decree Article 24(2) distinguishes sewage treatment facilities where daily sewage exceeds 2 m³ from septic tanks at 2 m³ or less.
Exactly 2.000 m³ and 2.00035 m³ fall on different sides of that test.
A rated population comparison cannot replace a check for a required change in facility type.
Special countermeasure or riparian protection area
For the areas covered by Article 24(4), installing flush toilets OR generating more than 1 m³ of sewage per day leads to the sewage-treatment-facility standard.
Either condition can apply independently.
Where flush toilets are installed, Q of 1 m³/day or less does not by itself establish eligibility for an ordinary septic tank.
Confirm an unknown protected-area designation for the actual address.
Inside a treatment area and connection to public treatment
The combined-sewer rule addresses septic tanks where flush toilets are installed.
Exceptions under Sewerage Act Article 34(1), including connection through a separate sewage sewer to public treatment, require the actual connection and treatment arrangement to be confirmed.
Being inside a treatment area alone does not establish an exception.
Other routes, such as an officially designated sewer-rehabilitation area with qualifying combined-sewer connection, also require authority confirmation.
Existing tanks: 120%, 150% and 200% are conditional review bands
Article 25 addresses extensions or changes of use that exceed existing capacity, including cases outside a treatment area where daily sewage exceeding 2 m³ calls for a new sewage treatment facility.
A septic-tank cleaning exception must not be treated as automatically removing a requirement to change facility type.
Sewage comparison bases, capacity ratios and internal-cleaning conditions for septic-tank exception review under Enforcement Decree Article 25(2)| Location | Sewage being compared | Capacity ratio | Minimum cleaning frequency |
|---|
| Outside treatment area | All sewage from flush toilets | ≤120% | Once every 6 months |
| Inside treatment area | Total sewage including the increase | ≤150% | Once every 9 months |
| Inside treatment area | Total sewage including the increase | >150% to ≤200% | Once every 6 months |
The outside-area numerator is toilet sewage; the inside-area numerator is total sewage including the increase.
Do not silently substitute the building’s total Q for toilet sewage.
Keep the review pending until the authority confirms compatible sewage and capacity units.
These septic-tank cleaning conditions do not automatically permit an overloaded sewage treatment facility.
A separate route involving demonstrated effluent compliance and an improvement-feasibility report requires a registered specialist and authority review.
Keep rated people, effective volume and daily capacity separate
A five-person product rating, 1.5 m³ of effective storage and 1.5 m³/day of treatment capacity are three different quantities.
Calculating N and Q does not verify an installed product’s detailed design or the measurement basis required for an Article 25 exception.
Enforcement Rule Annex 12 includes structural criteria for septic tanks using a septic-chamber process: ordinarily at least 1.5 m³ total effective volume, an additional minimum 0.5 m³ per five people beyond five, and at least 1.0 m³ where the treatment population is three or fewer.
These are structural/storage provisions, alongside requirements for filter media, compartments, depth and other details.
They must not be converted automatically into a daily sewage capacity.
Ask the designer to review the product certification, rated label, drawings and management records together with the calculated N and Q.
Rural homestay options, increases and evidence-based adjustments
- The ordinary rural-homestay rate is 35 L/m²/day with N=0.140A.
A house with exclusive residential area at most 100 m² AND at most two guest rooms may use the ordinary-lodging standard with cooking facilities after confirmation.
Exclusive residential area is a separate condition from the gross area used for the sewage calculation.
- For a calculation area of 100 m², the cooking-equipped lodging standard gives Q=2 m³/day and N=8, compared with Q=3.5 m³/day and N=14 under the ordinary homestay rate.
This comparison assumes the other eligibility conditions and the selected treatment have been confirmed.
It does not extend the homestay exception to tourist pensions.
- An increase of at least 10 m³/day prompts review under Enforcement Decree Article 35(1).
The calculator does not set a contribution amount or waive other obligations; check the local ordinance, past payment, approved baseline and scope of the administrative action.
- Notice Section 4(b) permits adjustments supported by objective investigation and forecasts of actual use.
A transitional provision also preserves sewage amounts for completed administrative actions calculated under former Notice No. 2021-59.
A current-formula before-change calculation can therefore differ from the legally approved historical figure.
- Select the actual use and include required common and outdoor business areas.
Using a lower-rate classification simply to reduce the result undermines the schedule.
Drainage arrangements, effluent performance, structure, product selection and permission to operate remain separate reviews.
Frequently asked questions
Should I enter the number of family members actually living there?
Residential baseline population follows the notice’s legal-room formula rather than the current resident register or visitor count.
Legal rooms can include bedrooms and other qualifying living or working spaces.
Use separate rows for homes with different room arrangements.
Does every one-room detached house count as two people?
The detached-house formula gives N=1.5 when R=1.
A separate two-person rule applies to an apartment-type home consisting of one legal room; it must not be extended automatically to detached houses.
Ask the designer and authority how the calculated population relates to a product’s rated population.
Why do cafés and general restaurants have equal N but different sewage volumes?
Both categories use N=0.175A, while their daily sewage rates are 35 and 60 L/m²/day respectively.
They are separate design indicators and should be calculated independently.
A business selling general-restaurant menus may require that classification even if its trading name includes café.
Does the lower rural-homestay rate apply automatically?
No.
A rural homestay in a house with exclusive residential area no greater than 100 m² and no more than two guest rooms may use the ordinary-lodging standard with cooking facilities.
Confirm both conditions and the permitted classification before selecting this option.
This homestay exception does not automatically apply to tourist pensions.
Can I keep a tank whenever the ratio is below 120% or 200%?
The ratios identify conditions for reviewing an existing septic-tank exception rather than an operating permit.
Confirm the treatment area, the sewage stream being compared, compatible capacity units, cleaning conditions and any requirement to change facility type.
Outside a treatment area, the 120% numerator is toilet sewage, not the building’s total sewage.
Can a five-person tank or an effective volume of 1.5 m³ be entered as 1.5 m³/day?
No.
Rated population, effective structural/storage volume and daily treatment capacity are different quantities.
This tool does not convert between them.
Keep the existing-facility exception review pending until the authority confirms an appropriate sewage amount and a capacity measured on the same basis.
Does an increase below 10 m³/day mean no sewer contribution is payable?
An increase of at least 10 m³/day triggers review of the threshold in Enforcement Decree Article 35(1).
Falling below that threshold does not waive obligations arising from other works, development or local rules.
Confirm the approved baseline and the administrative action covered by the assessment.
Can I use the result directly for a permit or change-of-use filing?
Use it as a preliminary calculation schedule for the designer and local authority.
The notice permits evidence-based adjustments, and a real installation also depends on effluent requirements, product performance, structure, drainage, notification and completion inspection.
Bring the building register, drawings and existing-facility records to the review.
Official sources and the rule date
The current instruments and annexes were checked directly through Korea’s National Law Information OPEN API on November 9, 2026.
Recheck amendments and address-specific application before design or filing.
The ministry name in these current instruments is the Ministry of Climate, Energy and Environment.
- Building-use sewage generation and septic treatment-population calculation notice: Notice No. 2025-165, effective 2025-10-01, Section 4 and annex
- Sewerage Act Article 34 and Article 35: MST 276803, effective 2025-10-01, installation, exceptions and building changes
- Sewerage Act Enforcement Decree Article 24, Articles 25 and 35: MST 286151, effective 2026-06-03, facility types, existing-facility conditions and increase threshold
- Sewerage Act Enforcement Rule Annex 12: MST 287447, effective 2026-06-22, structural, dimensional and performance criteria
Use the schedule to align design and construction quotations
Share the before-and-after uses, areas, homes, legal rooms, location assumptions, N, Q and unresolved questions together.
After authority confirmation, request quotations from at least two contractors using the same drawings and scope, including cleaning and operating conditions.
Use an actual installed rated population for cleaning schedules; use the confirmed sewage volume and increase for installation-cost and building-permit reviews.