Onn Point Civil Engineering

Feedback like this reflects exactly what Onn-Point is built on: technical depth, support that lasts the whole project, and a genuine commitment to getting schemes over the line. 👏

Complex sites are rarely solved in a single report. Constraints shift, consultees raise queries, and schemes get approved because someone stayed with the detail.

In an environment where drainage requirements are becoming more detailed and more demanding, our team's experience and responsiveness makes a difference.

Got a drainage design requirement? Get in touch:

📞 01902 585206 | ✉️ [email protected]

Standard 1 of the 2025 National Standards sets out where surface water can go, and in what order.

Reuse first. Then infiltration, an above-ground water body, a surface water sewer, with a combined sewer as a genuine last resort. Each tier has to be maximised as far as practicable before the design drops to the next.

Where strategies come unstuck is the evidence behind the choice:

📍 The standards are explicit that higher cost alone is not a reason to use a lower-priority destination

📍 A combined sewer connection will attract questions about everything ruled out above it

A BRE 365 test result is evidence. "Infiltration isn't viable" is not.

So ground investigation comes before design, not alongside it. Until you know whether tier two is available, nothing below it stands up.

Work the order, record the reasoning, and the strategy tends to follow.

📞 ✉️ 01902 585206 | [email protected]

Onn-Point recently produced the drainage strategy for a car park in Leeds, delivered for Leeds City Council. 🚗🅿️💧

Car parks are almost entirely impermeable, so every drop has to be accounted for. With infiltration not available, the route was a restricted discharge to the public sewer, at a rate agreed with the water company, and enough attenuation storage to hold back the difference.

The scheme was designed with a 35% climate change allowance on the 1 in 30 year event and 45% on the 1 in 100, with hydraulic simulation confirming no flooding in the 1, 30 or 100 year events.

What catches people out is that the condition often requires written confirmation from the sewerage undertaker that they accept the rate. Not a proposed figure, an agreed one. That correspondence is usually the slowest part of the process.

Fully impermeable site. Agreed discharge rate. Approved at planning.

📞 01902 585206 | ✉️ [email protected]

"We've done this before" is worth less in drainage than people expect.

A strategy that sailed through on the last site tells you very little about the next one, unless it happens to be nearby. Ground conditions, groundwater behaviour and what the local authority will accept all change with the postcode.

📍 Mercia Mudstone with made ground gives wildly variable results across a few metres

📍 London Clay usually rules infiltration out before design starts

📍 Chalk offers strong infiltration, then groundwater protection limits what's permitted

📍 Glacial till hides perched water that a summer test won't find

📍 Low-lying pumped catchments need peak groundwater evidence, not a single rate

Same season, same standards, entirely different problem.

Which is why the data has to come from the site rather than the region, and why the strategy is built around what the ground actually does.

📞 01902 585206 | ✉️ [email protected]

If your application creates new hard surfacing, expect a drainage condition. 📑🏡

Developments of 10 or more homes, or half a hectare and above, bring the lead local flood authority in as a formal consultee. But conditions land on smaller schemes too, and a single house on a tight plot can attract the same level of scrutiny.

What the authority wants to see:

- Where the water goes, worked through the SuDS hierarchy, with rejected options explained

- A restricted discharge rate, backed by calculation

- Storage sized for a 1 in 100 year storm plus a climate change allowance

- Where water runs if the system is overwhelmed

- A maintenance plan, with someone responsible for it

Most refusals and holding objections we see aren't design failures. The design is sound but the evidence behind it is thin.

Get the strategy in early and it shapes the layout. Leave it until after and the layout dictates the strategy.

📞 01902 585206 | ✉️ [email protected]

One greenfield site. A single luxury dwelling. Ground that would not take a drop of water. 🏡💧❌

Onn-Point recently delivered the surface water strategy for a residential scheme in Staffordshire, where percolation testing ruled out infiltration before design had properly started.

Without soakaways, every roof, driveway and parking area had to be captured, stored and released at greenfield rates through a single discharge point.

The approved strategy:

✔ Discharge to a watercourse on the site boundary at greenfield run-off rates

✔ Rainwater harvesting and raingardens capturing runoff at source

✔ Tanked permeable paving and attenuation storage providing on-site volume

✔ A detention basin attenuating flows before discharge

✔ Designed for a 1 in 100 year storm + 40% climate change allowance

Working as a treatment train rather than a single structure, each element takes volume out before the next has to deal with it. Hydraulic simulation confirmed no flooding across the 1, 30 and 100 year events.

Every option in the SuDS hierarchy was assessed and recorded, including those discounted and why. That evidence carries as much weight with the LLFA as the design.

The application was approved at planning.

📞 ✉️ 01902 585206 | [email protected]

Onn-Point has built a free soakaway size calculator into our website.

Enter your area, infiltration rate, rainfall depth, climate change allowance and proposed depth. It returns:

📦 Storage volume required

⛏️ Excavation volume

📐 Plan dimensions

⏱️ Half-drain time against the BRE 365 24-hour limit

The sizing is deliberately conservative, applying the full design rainfall with no credit for water soaking away during the storm.

It’s also a first check, not a design. A submission still needs site-specific FEH rainfall, multiple storm durations, groundwater levels and LLFA agreement.

But if you want to know whether a soakaway will fit on a plot before committing to a layout, it takes thirty seconds.

🔗 Check it out here - https://www.onn-point.co.uk/how-to-calculate-soakaway-size-for-effective-water-management/

Onn-Point was recently featured in BirminghamLive, setting out how we take a development from a drainage planning condition to a discharged one.

Four stages, in this order:

1️⃣ Percolation testing and ground investigation

2️⃣ Flood risk assessment

3️⃣ Drainage strategy and SuDS design

4️⃣ Discharging the planning condition

Skip the ground data and the strategy rests on an assumption. Skip the flood risk work and the levels may not hold. Get both right and the strategy has something solid to stand on when it reaches the LLFA.

Most delays we see aren’t design failures. They’re sequencing problems.

Great to see drainage covered as a development issue rather than a technical footnote. 👏

🔗 Read the full feature here - https://www.birminghammail.co.uk/special-features/perfect-your-project-four-easy-28076677

Earlier this year, Onn-Point produced a complex drainage strategy for the transformation of a Walsall office building into 11 residential flats.

Clay ground conditions ruled out infiltration from the start. On a tight urban site, that left one route: controlled attenuation and a restricted discharge.

The approved strategy:

✔ Discharge to an existing culverted watercourse

✔ Tanked permeable paving for on-site attenuation storage

✔ A Hydrobrake chamber restricting discharge from 11.2l/s to 3.5l/s

✔ Designed for a 1 in 100 year storm + 40% climate change allowance

✔ Full hydraulic modelling in Site3D to verify performance

Cutting the existing rate from 11.2l/s to 3.5l/s delivered a significant reduction in downstream flood risk, evidenced against LLFA expectations and the CIRIA SuDS Manual.

A constrained urban site. A measurable discharge reduction. Approved at planning.

📞 ✉️ 01902 585206 | [email protected]

The infiltration rate that gets a site through planning is measured in winter, not in July.

BRE 365 is clear on it. Test when soil moisture and groundwater are highest. October to March is that window.

It matters because the soakaway base must sit at least a metre above the maximum water table, and the design rate is the slowest result at the position the soakaway will occupy.

🗺️ Region changes everything. Mercia Mudstone with made ground behaves nothing like London Clay, chalk or glacial till.

⏳ Ground data, groundwater evidence, strategy, LLFA and water company consultation, then approval. Months, not weeks.

📐 Infiltration assumptions get built into layout and appraisal before anyone tests them. Confirming them now is a site visit. Correcting them in spring is a redesign.

740 percolation tests nationwide since 2019. One window that opens in October.

📞 01902 585206 | ✉️ [email protected]