Yes, in many cases a homeowner can dig a small soakage pit by hand — no earthmover required. However, whether you should depends on the size of the pit, your soil type, local council rules, and what the soakage pit is connected to. Get these factors wrong and you could end up with a failed drainage system, a flooded yard, or a compliance headache.
What Is a Soakage Pit?
A soakage pit (sometimes called a soakhole or soakage trench) is an underground void filled with coarse aggregate — typically 40 mm crushed rock — that allows stormwater or greywater to slowly percolate into the surrounding soil. They are commonly used for:
- Roof stormwater from downpipes
- Subsoil drainage from retaining walls or garden areas
- Overflow from rainwater tanks
They are not suitable for blackwater (sewage) disposal — that requires a septic system or connection to a public sewer, which is restricted work under the Plumbers Gasfitters & Drainlayers Board (PGDB).
Do You Need a Building Consent?
This is the first question to answer before you lift a spade. Under the NZ Building Code and the Building Act 2004, some minor drainage work is exempt from consent under Schedule 1, but the exemptions have conditions and vary by council. As a general guide:
- Soakage pits for roof stormwater on a residential property are often exempt if they meet certain size and setback requirements.
- Work must still comply with NZ Building Code clause E1 (Surface Water) and your local council's stormwater management rules.
- Some councils require a resource consent if you are discharging to ground in a sensitive catchment or near a waterway.
Always check with your local council before you start. Do not assume your project is exempt — the rules differ between territorial authorities. You can find general guidance at MBIE Building Performance.
Can You Dig It by Hand?
For a typical residential soakage pit — say, 1 m wide × 1 m long × 1–1.5 m deep — hand digging with a spade and a long-handled shovel is entirely feasible for a reasonably fit person, provided:
- Soil is workable — sandy loam, light clay, or gravelly soils are manageable by hand. Heavy clay, rock, or compacted fill may require a mini-excavator or hired plant.
- No underground services are present — always call Dial Before You Dig (0800 248 344) or check with your council before breaking ground. Striking a gas line, water main, or power cable is dangerous and costly.
- The pit is not too deep — working in excavations deeper than 1.5 m carries serious collapse risk. WorkSafe NZ requires safe shoring or battered sides for deeper excavations. For most soakage pits, staying under 1.5 m is both safer and sufficient.
- Groundwater is not an issue — if water starts seeping in as you dig, the water table may be too high for a soakage pit to work effectively at that location.
Soil Permeability Testing
Before you dig, it pays to do a simple percolation (perc) test to confirm your soil will actually absorb water at a useful rate. Dig a small test hole, fill it with water, let it drain, then refill and time how long it takes to drain. Your council or a drainlayer can advise on acceptable rates. Soakage pits in very slow-draining clay soils often fail and can cause waterlogging or even undermine foundations.
How to Construct a Basic Soakage Pit
- Mark out and check for services — use spray paint and confirm with Dial Before You Dig.
- Dig to the required depth — keep sides as vertical as possible and pile spoil well away from the edge.
- Line with geotextile fabric — wrap the pit in a permeable geotextile (available from Mitre 10 or Bunnings) to prevent soil migration into the aggregate.
- Fill with clean aggregate — use 40 mm crushed rock or similar free-draining material, leaving space at the top for an inlet pipe.
- Install the inlet pipe — typically 100 mm PVC stormwater pipe from the downpipe or drain.
- Cap with geotextile and topsoil — fold the fabric over the top of the aggregate, then backfill with soil and reinstate the surface.
Key Setback Rules (check with your council)
| From | Typical minimum setback |
|---|---|
| House foundation | 3 m |
| Boundary | 1.5–3 m |
| Waterway or bore | Check with council |
| Septic tank/field | Check with council |
These are indicative only — your council may require different setbacks.
What About Connecting to Existing Drains?
If your soakage pit needs to connect to an existing stormwater drain or any part of the drainage system that serves the building, that work may be restricted building work or restricted drainlaying work requiring a licensed drainlayer. Check with the PGDB and your council if you are unsure.
When to Call a Professional
Consider hiring a qualified earthmover or drainlayer if:
- The soil is heavy clay, rock, or compacted fill that is too hard to dig by hand
- The pit needs to be deeper than 1.5 m
- You are connecting to an existing drainage system
- Your council requires inspections or a consent
- You are unsure about soil permeability or groundwater levels
- The area is near foundations, retaining walls, or boundaries
A professional earthmover can complete a job in a fraction of the time and ensure the pit is correctly sized and positioned. You can find qualified local earthmovers and drainlayers on tradedirectory.co.nz.
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