Inter-Allotment Drainage Design NSW: How Engineers Manage Shared Stormwater

Inter-allotment drainage design NSW is something many homeowners and developers only hear about once they are already mid-project. By that stage, the survey is done, the plans are drawn, and nobody mentioned that stormwater from their site has to travel through a neighbour’s land before it can reach the council drain.

At R I Eng Pty Ltd, we have seen this situation many times. We design IAD systems for houses, duplexes, townhouses, attached dwellings, and granny flats across NSW — and we sort it out before it becomes a delay on your project.

📞 Call: +61 0451 452 932
💬 WhatsApp: wa.me/610451452932
🌐 Get a Quote → riengineering.com.au

What Is Inter-Allotment Drainage?

Inter-allotment drainage (IAD) is a stormwater pipe system installed beneath private land. It carries runoff from one property through an adjacent allotment to a legal discharge point — usually the council street drainage network.

In NSW, inter-allotment drainage design becomes necessary when a site cannot drain directly to the street. This happens when the land sits lower than the kerb, or when the natural ground fall runs away from the public system rather than toward it.

The IAD system includes pipes, pits, and connections. It runs through a drainage easement — a legal right-of-way registered on the title of the downstream land. This keeps the pipe accessible for maintenance over time.

When Does a NSW Development Need an IAD Design?

Not every project requires inter-allotment drainage. Your site will likely need an IAD civil design in NSW when:

  • Your property is lower than the street and cannot gravity-drain to the kerb
  • A newly created lot cannot reach the council system without crossing another allotment
  • Your duplex, townhouse, or granny flat increases stormwater runoff that cannot stay on-site
  • Council’s drainage map shows your land sits within a catchment that relies on a registered IAD easement

A civil engineer checks the site survey, natural catchment boundaries, and council infrastructure to confirm whether IAD applies. This is also the stage where we review your stormwater drainage design as a whole — including whether an on-site detention system is also required for your development.

How R I Eng Designs an IAD System

Our inter-allotment drainage design NSW process follows these steps.

First, we review the survey and identify all hard-surface areas contributing stormwater runoff.

Then, we calculate the design flow using the Rational Method — based on catchment area, soil type, and the council’s nominated storm event (typically the 1-in-20-year or 1-in-100-year storm).

Next, we size the IAD pipes using Manning’s equation to confirm the pipe carries the design flow without surcharging.

We then establish pit locations and invert levels to create a workable fall through the easement, accounting for minimum cover depths and existing services.

Finally, we produce the civil design drawings — pipe sizes, pit schedules, invert levels, and easement dimensions — ready for council submission.

🟢 We turn around most IAD designs in 2–4 business days.
📞 Call: +61 0451 452 932
💬 WhatsApp: Chat with us
🌐 Request your IAD design → riengineering.com.au

IAD Design for Houses, Duplexes, and Granny Flats

Most inter-allotment drainage design work in NSW comes from residential developments. R I Eng Pty Ltd regularly designs IAD systems for:

  • Single dwelling houses on low-lying lots
  • Duplex and dual-occupancy developments where one lot sits behind the other
  • Attached dwellings and townhouse projects with shared drainage paths
  • Granny flat additions on sloping blocks where extra runoff must be managed

Each project has different site conditions. We tailor the pipe sizing, pit placement, and easement design to the actual survey data — not a generic template.

For more on how we handle drainage across different residential project types, visit our drainage design services page.

Frequently Asked Questions

Q: What is inter-allotment drainage in NSW?
A: Inter-allotment drainage is a pipe system that carries stormwater from one property through an adjacent allotment to reach a lawful discharge point. It runs through a registered drainage easement on the downstream land. NSW Government planning guidelines outline when this is required for development.

Q: When do I need an IAD design for my property?
A: You need an inter-allotment drainage design when your site cannot drain directly to the street or council system. A civil engineer confirms this from your site survey and drainage levels.

Q: Who designs inter-allotment drainage in NSW?
A: A civil engineer designs the IAD system — pipe sizing, pit layouts, and invert levels. R I Eng Pty Ltd provides IAD civil design in NSW for residential projects from single houses to townhouse developments.

Q: What is a drainage easement and do I need one?
A: A drainage easement is a legal right registered on the title of the property the IAD pipe passes through. Your solicitor or surveyor handles easement registration. R I Eng designs the pipe system and produces the engineering drawings to support it.

Q: How long does IAD design take?
A: Most inter-allotment drainage design NSW projects take two to four business days from receipt of a current survey. Urgent options are available.

Need Inter-Allotment Drainage Design in NSW?

R I Eng Pty Ltd designs IAD systems for houses, duplexes, townhouses, attached dwellings, and granny flats across NSW. We deliver pipe sizing, pit layouts, invert levels, and drainage easement design drawings — ready for council submission.

📞 Call: +61 0451 452 932
💬 WhatsApp: wa.me/610451452932
📧 Email: riengineering2155@gmail.com
🌐 Visit riengineering.com.au →