Free to size. SAB-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. You'll need your percolation test and the site's rainfall depths.
When SAB approval applies
Schedule 3 to the Flood and Water Management Act 2010 has been in force in Wales since 7 January 2019. Construction work with drainage implications needs the SuDS Approving Body's approval before it starts, unless the area it covers is under 100 m². That is the construction area, not the drained area: Welsh Government's advice note adds roofs, drives, patios and extensions together. A single house or an extension under the threshold stays a Building Regulations Part H job — for that, the standard SoakMonkey design to BRE Digest 365 is the right one.
What the SAB checks that Building Control doesn't
| Building Regulations, Part H | SAB — Statutory SuDS Standards | |
|---|---|---|
| Design storm | 1 in 10 year (BRE Digest 365) | 1 in 30 and 1 in 100 year, with climate change (S2; G2.34) |
| Infiltration rate | The lowest test result | The test result divided by a factor of safety (G1.17, Table G1.1) |
| Half-emptying | Within 24 hours | Within 24 hours for a 1 in 10 or 1 in 30 year design; longer above that only with the SAB's agreement (G1.11) |
| Slow ground | — | Soakaways usually need about 1 × 10⁻⁵ m/s or more (G1.15) |
| Distance from buildings | At least 5 m | At least 5 m, unless a geotechnical assessment supports less (G1.20) |
| Groundwater | Clear of the water table | At least 1 m of unsaturated ground below the base (G3.32) |
Infiltration also sits second in the Standard S1 hierarchy — after collecting the water for use, and ahead of discharge to a watercourse or sewer — so a soakaway that works is the answer the SAB wants to see.
The factor of safety (Table G1.1)
The measured infiltration rate is divided by this before the soakaway is sized, so it has a large effect.
| Area drained | No damage or inconvenience | Minor inconvenience (e.g. surface water on car parking) | Damage to buildings or structures, or major inconvenience |
|---|---|---|---|
| Under 100 m² | 1.5 | 2 | 10 |
| 100–1000 m² | 1.5 | 3 | 10 |
| Over 1000 m² | 1.5 | 5 | 10 |
Rainfall and climate change
The Standards give no percentage themselves; they point to current guidance. Welsh Government's climate change allowances for applications from 1 April 2026 give peak rainfall uplifts of +30% (central) and +40% (upper) for 2061–2125, and say drainage should be designed so there is no increase in site run-off when assessed against the upper allowance. SoakMonkey defaults to +40%; change it if your SAB asks for something else, and tick urban creep if it wants that too.
For the rainfall itself, some SABs expect Flood Estimation Handbook (FEH) depths for the site (Conwy's local guidance asks for them) rather than the older FSR figures. In SAB mode SoakMonkey takes your site's 1 in 30 and 1 in 100 year depths for storms from 15 minutes to 24 hours, adds the climate-change allowance and finds the critical duration for each.
What to send with the application
Welsh Government's 2026 statutory guidance asks for a plan of the construction area and its drainage, and information on how the work will comply with the Standards. For the infiltration calculation, the All-Wales application form guidance lists the critical-duration storms, the climate change and urban creep allowances, the stated factor of safety and infiltration rate, and the time to half-empty. SoakMonkey's SAB report sets out all of those for the 1 in 30 and 1 in 100 year events, with your percolation test. The same guidance also asks for 1 and 2 year results, which matter mainly for pipe networks — check whether your SAB wants them for a single soakaway.
How SoakMonkey does it
- Pick Wales — SuDS Approving Body in step 1 (the button above does it for you).
- Enter the drained area, then the site's 1 in 30 and 1 in 100 year rainfall depths, and choose the consequence of failure.
- Enter your percolation test. The tool divides the rate by the Table G1.1 factor, sizes the soakaway for the design storm you choose, and checks the half-empty time.
- Size it on the 1 in 30 year storm instead and it tells you how much of the 1 in 100 year storm overflows — the volume you must manage above ground without flooding any building.
Frequently asked questions
When does a soakaway in Wales need SAB approval?
When the construction work covers 100 m² or more. Since 7 January 2019, new development in Wales with drainage implications needs the approval of the SuDS Approving Body (SAB) — the local authority — before work starts, unless the area covered by the construction work is under 100 m². Welsh Government's advice note counts roofs, drives, patios and extensions together. Below 100 m², the drainage is a Building Regulations Part H matter.
Is a Building Regulations soakaway design enough for the SAB?
Usually not. A Part H design to BRE Digest 365 is sized for the 1 in 10 year storm with no factor of safety. The Statutory SuDS Standards look at the 1 in 30 and 1 in 100 year storms with climate change, divide the infiltration rate by a factor of safety from Table G1.1, and add siting rules: at least 5 m from a building and at least 1 m of unsaturated ground above the highest groundwater.
What factor of safety does the SAB use for a soakaway?
Table G1.1 of the Statutory SuDS Standards sets it from the area drained and the consequence of failure: 1.5 where failure would cause no damage or inconvenience; 2 (under 100 m²), 3 (100–1000 m²) or 5 (over 1000 m²) for minor inconvenience such as surface water on car parking; and 10 where failure could damage buildings or structures or cause major inconvenience. The measured infiltration rate is divided by the factor before the soakaway is sized.
What climate change allowance should I use in Wales?
Welsh Government's allowances for applications submitted from 1 April 2026 give peak rainfall uplifts of +20% (central) and +35% (upper) up to 2060, and +30% and +40% for 2061–2125. The guidance says drainage should be designed so there is no increase in site run-off when assessed against the upper allowance. Check what your SAB expects: some local guidance uses the central figure plus 10% urban creep.
Does the SAB accept BRE Digest 365?
As a design approach, yes: the Statutory SuDS Standards say infiltration design should follow the SuDS Manual, RP 156, BRE 365 or another recognised approach. What changes is what goes into it — the 1 in 30 and 1 in 100 year rainfall with climate change, and a factor of safety — and some SABs expect site rainfall from the Flood Estimation Handbook (FEH) — Conwy's local guidance, for one — rather than the older FSR figures BRE 365 was written around.
How much does SAB approval cost?
The application fee is £350 plus £70 for each 0.1 hectare or part of it, up to 0.5 ha, so a site under 0.1 ha pays £420. Each inspection is £168. The SAB has seven weeks to decide, or twelve where an environmental impact assessment is needed.
Sources
- Welsh Government, Statutory standards for sustainable drainage systems (2018) — Standards S1 and S2, G1.10–G1.20, Table G1.1, G2.34, G3.31–G3.32.
- Welsh Government, Sustainable drainage (SuDS) statutory guidance (March 2026).
- Welsh Government, Flood Consequences Assessments: climate change allowances (for applications from 1 April 2026).
- Welsh Government, Advice note: SAB applications for single dwellings, extensions and parking and access areas (July 2022).
- SI 2018/1074 (exemptions) and SI 2018/1075 (fees).
Related: how to do the percolation test, the BRE Digest 365 method, and soakaways for a building warrant in Scotland.