A drainage plan can appear complete on an architectural drawing and still be unsuitable for authority submission. Missing invert levels, an unverified outlet connection, insufficient drain capacity, or clashes with structural works can all delay the review process. A PUB drainage design submission must demonstrate more than the intended pipe route. It must show that the proposed drainage system manages surface runoff safely, connects appropriately to the public drainage network, and is coordinated with the actual site conditions and building works.

For property owners, developers, architects, and contractors, the most effective approach is to address drainage early – before foundations, driveway levels, landscaping, and external works are fixed. Changes made after architectural plans or construction works have progressed are usually more disruptive, more expensive, and harder to coordinate.

PUB Drainage Design Submission Requirements

A drainage submission is assessed against the applicable drainage requirements, site constraints, and the proposed scope of development. The design should account for how rainwater moves across the site, where it is collected, the capacity and levels of the proposed system, and how it discharges without creating a drainage, flooding, or maintenance concern.

The required level of design detail depends on the project. A landed house redevelopment, an addition and alteration project, a commercial fit-out with external works, and an industrial development will not necessarily have the same drainage scope. A modest renovation may only involve localized drainage modifications. A redevelopment with new building footprints, basements, driveways, boundary walls, and landscaped areas requires a much broader review of site drainage and outfall arrangements.

The design team should first establish whether there are existing public drains, roadside drains, internal drains, or downstream systems that affect the proposed work. Existing records are useful, but they must be checked against survey information and actual site observations where possible. An assumed drain location or invert level can create a major issue if it is later found to differ from the approved design basis.

Drainage planning must also be coordinated with the relevant Qualified Person and Professional Engineer roles for the project. The appointment, endorsement, and submission responsibilities depend on the nature of the development and the regulatory scope. Clear allocation of these responsibilities prevents a common problem: drawings that are technically prepared but cannot proceed because the required professional coordination or endorsement has not been established.

Start With the Existing Site and Levels

Levels govern drainage performance. Before finalizing a layout, the project team should understand the existing ground levels, proposed finished floor levels, driveway gradients, road levels, drain invert levels, and intended discharge point. This information determines whether runoff can drain by gravity and whether the external works create low points that could trap water.

A site with a steep fall may need interception drains, carefully placed gullies, or stepped drainage arrangements to prevent runoff from flowing toward a building entrance or neighboring property. A relatively flat site may require more deliberate grading and adequate collection points to avoid ponding. Basement ramps, sunken courtyards, loading areas, and lowered planter zones need particular attention because they can become concentrated collection areas during intense rainfall.

The architectural, civil, structural, and landscape designs should use a consistent level strategy. For example, a proposed driveway may work geometrically between the road and garage, but its crossfall may direct water toward the building if drainage channels are not included. Similarly, a perimeter drain can conflict with retaining walls, foundation edges, service ducts, or tree pits if it is treated as an afterthought.

A reliable design typically begins with a current topographical survey, relevant drainage records, and a review of the proposed site plan. Where existing drains or discharge points are uncertain, investigation may be necessary before the design is committed. This upfront work is usually less costly than redesigning completed external works.

Design the Drainage System as One Connected Network

The drainage design should show the complete path of runoff: collection at roofs, paved areas, landscaped zones, and other surfaces; conveyance through gutters, downpipes, channels, pipes, or drains; and discharge to an approved downstream point. Each component must work with the next. Oversizing a local drain does not resolve a restriction at the downstream connection.

Hydraulic capacity should be considered alongside physical buildability. Pipe sizes, drain dimensions, gradients, silt traps, inspection chambers, and connection details need to be practical for installation, accessible for maintenance, and compatible with site levels. Where changes in direction, level, or drain size occur, the detailing should make the drainage route clear to both reviewers and construction teams.

Roof drainage warrants careful coordination. Downpipes should not discharge in a way that erodes landscaped areas, overwhelms a narrow channel, or creates uncontrolled flow across walkways. For larger roofs or complex building forms, the collection layout should be reviewed together with parapet levels, roof falls, overflow provisions, and the location of M&E equipment.

Drainage systems also need separation from sanitary systems unless a specifically approved arrangement applies. Site teams should not rely on generic plumbing assumptions. The submitted drawings must clearly distinguish surface water drainage, sanitary drainage, and any other underground services to reduce the risk of incorrect construction or conflicts during inspection.

Documents That Support a Clear Submission

A complete PUB drainage design submission is not simply a plan with drain lines. The documentation should allow the reviewing authority to understand the existing conditions, proposed works, design intent, and connection arrangement without relying on assumptions. Depending on the project, a coordinated package may include:

• Site and drainage plans showing boundaries, road reserves, existing and proposed drains, collection points, pipe routes, flow directions, and discharge locations.
• Drainage profiles, sections, and details identifying dimensions, gradients, invert levels, cover levels, chambers, channels, gratings, and construction interfaces.
• Hydraulic calculations or supporting design checks where required by the project scope and applicable drainage criteria.
• Endorsement documents, forms, and professional declarations prepared by the appointed QP or PE, where applicable.

Drawings should use legible scales and consistent references. The site plan, drainage plan, architectural layout, and structural plans must agree on building footprint, boundaries, levels, retaining structures, and proposed external works. Inconsistencies between disciplines are a frequent reason for clarification requests because they make it difficult to confirm what will actually be constructed.

Coordinating a PUB Drainage Design Submission

Drainage cannot be designed in isolation. It is affected by architectural planning, geotechnical conditions, structural foundations, driveway geometry, landscape design, utility routes, and construction sequencing. On constrained sites, the available corridor for drainage may be limited by boundary setbacks, underground tanks, electrical ducts, sewer lines, piles, and existing trees.

Early coordination is particularly valuable for addition and alteration works. Existing buildings may have undocumented drains, altered external levels, or drainage routes that do not match older drawings. Before adding a new extension, covered area, deck, or driveway, the team should verify whether the existing system can be retained, modified, or needs to be upgraded.

Temporary works may also affect drainage performance. Excavation, demolition, temporary access, stockpiling, and construction hoarding can obstruct existing flow paths or introduce sediment into drains. A compliant permanent design does not remove the contractor’s need to manage site runoff and protect drainage infrastructure throughout construction.

Common Issues That Cause Delays

Many submission issues are preventable. One recurring problem is an incomplete connection proposal. A plan may show a pipe extending toward a public drain without proving the actual connection point, levels, or required interface works. Another is a mismatch between the architectural finished levels and the drainage profile, making the proposed gradient physically impossible.

Uncoordinated boundaries are also problematic. Drainage reserves, road widening requirements, easements, and public infrastructure constraints can affect where drains may be constructed. A drain located within a future affected area or across an unresolved boundary condition may require redesign.

Insufficient detail at building entrances and ramps is another concern. These areas need clear falls, collection points, and discharge routes. A note stating that the area will be graded to drain is not a substitute for showing levels and drainage components. The same applies to planter areas and paved courtyards, where the finished surface design can significantly alter how runoff behaves.

Finally, late design changes should be controlled. Revisions to the building footprint, car porch, retaining wall, or landscape layout can change the drainage catchment and block an approved route. The drainage designer should review these changes before revised drawings are issued for construction or resubmission.

A Practical Submission Sequence

An efficient workflow starts with records, survey data, and a site review. The design team can then establish the drainage concept, identify the proposed outfall, and coordinate critical levels with the architectural and structural plans. Once the layout is stable, detailed drawings and calculations can be prepared for the required professional review and submission.

This sequence is not identical for every project. A simple external alteration may move quickly if existing drainage is verified and the works are limited. A redevelopment, site with complex grades, or project involving road interface works needs more time for surveys, technical coordination, and authority clarification. Attempting to compress these steps by submitting incomplete information often creates a longer approval cycle.

AEC Technical Advisory can support this process through coordinated engineering review, technical documentation, professional endorsement, and authority submission management. The objective is to provide a drainage design that is not only suitable for PUB review, but also buildable and coordinated with the wider project.

The best time to resolve drainage risk is when a line on a drawing can still be moved. Confirm the outfall, verify the levels, and coordinate the disciplines before site work makes every adjustment more difficult.