ProjectCalcWhat Part H covers
Approved Document H (2015 edition) contains six requirements: H1 foul water drainage, H2 wastewater treatment systems and cesspools, H3 rainwater drainage, H4 building over sewers, H5 separate systems of drainage, and H6 solid waste storage.
Where surface water is allowed to go
The order of preference is set out at H3, paragraph 3.2: "Surface water drainage should discharge to a soakaway or other infiltration system where practicable." Discharge to a watercourse may need a consent from the Environment Agency, which may limit the rate of discharge (3.3). Where no other form of outlet is practicable, discharge should be made to a sewer (3.4).
For design, the document allows an assumed rainfall intensity: 0.014 litres/second/m² may be assumed for normal situations (3.8). Where low levels of surface flooding could flood buildings, the intensities should instead be taken from BS EN 752-4.
Soakaway sizing — paragraph 3.27
This is the paragraph that decides whether a soakaway works:
- Soakaways should be designed to a return period of once in ten years, using storms of different durations to find the duration that gives the largest storage volume.
- For small soakaways serving 25 m² or less, a design rainfall of 10 mm in 5 minutes may be assumed to give the worst case.
- For soakaways serving larger areas, the design should follow BS EN 752-4 or BRE Digest 365 Soakaway design.
- Soakaways for areas of less than 100 m² are generally formed from square or circular pits, filled with rubble or lined with dry-jointed masonry or perforated ring units.
- Where the ground is marginal, overflow drains can be acceptable.
The 5-metre rule, and what else rules a soakaway out
Paragraph 3.25 is a list of places an infiltration device should not be built. It should not be built:
- within 5 m of a building or road, or in areas of unstable land;
- in ground where the water table reaches the bottom of the device at any time of the year;
- so close to any drainage fields, drainage mounds or other soakaways that the overall soakage capacity of the ground is exceeded or the effectiveness of a drainage field is impaired;
- where contamination in the runoff could pollute a groundwater source or resource.
Percolation testing
The capacity of the soil has to be measured, not guessed. Paragraph 3.28 requires percolation tests to determine the capacity of the soil, carried out in accordance with paragraphs 1.34 to 1.38 of Approved Document H2, and gives the relationship between the soil infiltration rate and the value derived from the test. The test method itself (H2, paragraph 1.34) is specific: a hole 300 mm square, excavated to a depth 300 mm below the proposed invert level of the effluent distribution pipe, enlarged above that level where deep excavation is needed, or a modified procedure using a 300 mm earth auger.
Drain sizes and gradients
| Item | The rule |
|---|---|
| Minimum drain diameter | Drains should be at least 75 mm diameter; surface water sewers serving more than one building should have a minimum size of 100 mm — 3.14. |
| 75 mm and 100 mm rainwater drains | Laid at not less than 1:100 — 3.15. |
| 150 mm drains and sewers | Laid at gradients not less than 1:150 — 3.15. |
| 225 mm drains | Laid at gradients not less than 1:225 — 3.15. |
| Traps | Surface water drainage connected to combined sewers should have traps on all inlets — 3.7. |
Drainage fields
Where a treatment system discharges to ground, paragraph H2 1.40 requires drainage fields or mounds to be constructed using perforated pipe laid in trenches of a uniform gradient not steeper than 1:200, and H2 1.41 requires pipes to be laid on a 300 mm layer of clean shingle or broken stone graded between 20 mm and 50 mm.
What this changes about the job
A soakaway is not a hole filled with rubble at a convenient corner. Its size comes from the area it drains and the infiltration rate of the soil, its position is constrained by the 5 m separation from buildings and roads, and the ground has to be tested rather than assumed. If a soakaway cannot be sited within those constraints, the fallback in the document is a consented discharge to a watercourse or a connection to a sewer — a different job with different costs.
The editions this document cites
These are British and European standards this approved document names. The edition column is the one the document itself cites; the last column is what BSI catalogues today (22 September 2026). Where the two differ, the document is citing an edition that has since been replaced or withdrawn — that is normal for an approved document, and it is why the current edition is listed beside it rather than instead of it.
| Standard | Title as published | Edition cited by the approved document | Current edition (BSI) |
|---|---|---|---|
| BS EN 206-1 | Concrete — Specification, performance, production and conformity. Part 1: Performance, requirements, factory production control and assessment criteria for individual values | BS EN 206-1 — cited by Approved Document C as "BS EN 206-1 Method of specifying and guidance for the specifier" and by Approved Document H as "BS EN 206-1:2000" | BS EN 206-1:2026 — Current (31 March 2026) |
| BS EN 752 | Drain and sewer systems outside buildings. Sewer system management | BS EN 752-4:1998 — cited by Approved Document H, which also cites BS EN 752 Parts 3 and 6 | BS EN 752:2017 — Current, Under Review (the part-numbered structure is gone) |
| BS EN 1295-1 | Structural design of buried pipelines under various conditions of loading — General requirements | BS EN 1295 — cited by Approved Document H with no edition given | BS EN 1295-1:2019 — Current |
| BS EN 1610 | Construction and testing of drains and sewers | BS EN 1610 — cited by Approved Document H with no edition given | BS EN 1610:2015 — Current, Under Review |
Source: BSI catalogue records read at knowledge.bsigroup.com on 22 September 2026. Each title, designation, status and date in the last two columns was read from the BSI catalogue page for that standard.
Last verified 22 September 2026. Every requirement quoted on this page was read from the primary source listed below on that date, not from a summary. If a figure could not be confirmed in the primary source it is not on this page.
Where these figures come from
- Approved Document H: Drainage and waste disposal, 2015 edition (Requirements H1-H6; paragraphs 3.2-3.8, 3.14-3.15, 3.25-3.30 and H2 1.34, 1.40-1.41) — GOV.UK publications, "Drainage and waste disposal: Approved Document H", checked 22 September 2026.
- BSI catalogue (“BSI Knowledge”) — product record for each standard listed above: title, current edition and status — knowledge.bsigroup.com, checked 22 September 2026.
Guidance only, not advice
This page is a plain-English summary of published regulations and standards, written to help you plan a job. It is not professional advice, it is not a substitute for the actual document, and it is not a building-control decision.
Building regulations are enforced by your local authority (or an approved inspector) and the facts of a particular job can change the answer. Confirm with your local building control before you start work, and where a job is structural, get it designed by a suitably qualified person. If this page and the source document disagree, the source document wins.