Installing a small soakage pit for your downpipe runoff is something many NZ homeowners can tackle themselves — but only if the work meets your local council's requirements, the soil conditions are suitable, and the installation stays within the rules around building consents and drainage setbacks. Read on to understand what's involved before you grab a spade.
What Is a Soakage Pit?
A soakage pit (sometimes called a soakhole or soak pit) is an underground void — typically filled with coarse gravel or rubble — that allows stormwater from your roof downpipes to slowly disperse into the surrounding soil. They are a common and practical solution on properties where connecting to a kerb-and-channel or stormwater drain isn't straightforward.
Do You Need a Building Consent?
This is the first question to answer. Under Schedule 1 of the Building Act 2004, certain minor drainage work is exempt from building consent — but the exemptions have conditions and vary by council. A small soakage pit for roof stormwater may qualify as exempt work in many cases, but you should always confirm with your local council before starting. Some councils have their own stormwater bylaws that impose additional requirements.
For guidance on what generally requires consent versus what is exempt, MBIE Building Performance is the authoritative source. Don't rely on a neighbour's experience — rules differ between territorial authorities.
Is Drainlaying Restricted Work?
Yes — in New Zealand, drainlaying is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This means that connecting any new drain to an existing drainage system (including a public stormwater drain or an on-site drainage network) must be carried out by, or under the supervision of, a licensed drainlayer.
However, a simple soakage pit that is entirely self-contained — where you are simply redirecting a downpipe into a gravel-filled pit with no connection to any existing drain — may fall outside the restricted work definition. The key distinction is whether you are working on or connecting to a drain as defined by the Act. If you are unsure, check with the Plumbers Gasfitters & Drainlayers Board or consult a licensed drainlayer. Getting this wrong can result in fines and costly remediation.
Checking Your Soil's Suitability
A soakage pit only works if your soil can actually absorb water. Clay-heavy soils drain very poorly and can cause the pit to fill up and overflow, potentially directing water toward your foundations or a neighbour's property. Sandy or loamy soils generally perform well.
A simple percolation test (perc test) will tell you how quickly water drains from a test hole. Dig a hole roughly 300 mm square and 300 mm deep, fill it with water, and time how long it takes to drain. If it takes more than a few hours to empty, soakage may not be viable on your site. BRANZ has useful guidance on stormwater management and soil drainage considerations for NZ conditions.
Key Setbacks and Siting Rules
Where you place your soakage pit matters. Common requirements include:
- Minimum 1.5–3 m from any building foundation (check your council's specific rules)
- Minimum 1.5 m from property boundaries
- Well clear of any septic tank, effluent field, or potable water supply
- Away from slopes where saturated soil could cause instability
- Not in areas of known high groundwater
Always check with your council for the exact setback distances that apply in your area.
How to Build a Basic Soakage Pit
If you've confirmed consent isn't required, the soil is suitable, and you're not connecting to an existing drain, here's a general approach:
- Dig the pit — A typical small pit might be 600–900 mm in diameter and 900–1200 mm deep, but size it to handle your roof's catchment area.
- Line the sides with geotextile fabric to prevent soil migration while allowing water to pass through.
- Fill with clean, coarse aggregate (40 mm crushed rock is commonly used) to within about 200–300 mm of the surface.
- Install an inlet pipe from your downpipe into the top of the pit, ideally with a simple sediment trap or leaf guard to reduce blockages.
- Cap the top with geotextile, then backfill with topsoil and reinstate the surface.
You can find aggregate, geotextile, and fittings at Bunnings, Mitre 10, or PlaceMakers.
Safety Note
Dig carefully — always call Before U Dig (0800 248 344) to check for underground services before excavating. Cave-in risk is real in loose soils; never enter an unsupported excavation — check WorkSafe NZ guidance for the safe depth limits that apply to your soil conditions. Review safe excavation guidance from WorkSafe NZ.
When to Call a Professional
Call a licensed drainlayer if:
- Your downpipe needs to connect to an existing stormwater drain or council network
- Your council requires consented work or an inspection
- Your soil is clay-heavy or the site has poor drainage
- The pit needs to be larger than a simple residential soakhole
- You are on a sloped site or near a retaining wall or foundation
- You are unsure about any aspect of the rules
A licensed drainlayer can also certify the work, which protects you when it comes time to sell your home. You can find qualified local drainlayers on tradedirectory.co.nz.
Certifying Drainlayer
