Soakaway Design Service

We offer a bespoke design service for soakaways serving domestic single properties, multiple properties or larger developments. Our Chartered Civil Engineers with years of design experience and unique in house design software allow us to offer a site specific soakaway design completed within 2 working days for a wide variety of situations.

Our service is perfect for homeowners who need a professional soakaway design compliant with BRE 365 and planning conditions, or for professional designers who need a fast turnaround for more larger or more complex developments.

Simply complete the below form and we will get back to you with payment information and advise if we require any further information. Guidance on each input is included further down this page.

Single Property - £100
Up to 5 Properties - £200
Larger Developments - £500 (per soakaway if more than one required)

Our design package includes;

  • Full calculation output from our design software
  • Typical construction drawings in PDF and DWG format

    Form Input Guide

    Catchment Area

    This is the plan area in m2 of land which will drain into the proposed soakaway. For a roof this is the plan area of the roof (not adjusted for the slope). For a driveway this is the area of the tarmac. This field is used to calculate the rainfall parameters and therefore is required.

    Runoff Coefficient

    This coefficient (in percent) describes the amount of rainfall which runs off the surface during a rainfall event. For hardstanding areas this coefficient can be conservatively estimated at 90%-100%. For soft ground or grassed areas a lower value is sometimes assumed. This value can be left blank and a conservative value of 100% will be assumed. Further details can be found here if a more accurate value is required.


    A location is  required to calculate the rainfall characteristics of the site. This can be entered as a postcode or a place name. Please include a county or other identifier for smaller places. This field is required.

    Soil Infiltration Rate

    This is critical for the performance of the soakaway so must be chosen with care. Ideally a percolation test is required to ascertain an infiltration rate for detailed soakaway design. In some cases this might not be practical and a value can be estimated from other parameters such as soil characteristics but this will typically lead to a conservative value being chosen. We can assist with an estimated value if required but this is not recommended and subsequent designs are likely to be conservative and at your risk! Further details can be found here.

    Return Period

    This determines the likelihood of the design runoff values being exceeded and the soakaway being at risk of flooding. An appropriate value will depend on the consequences of short periods of flooding, a conservative value will lead to a larger more expensive soakaway. The planning authority may have specified a value for return period. Further details are included here.

    Further Details

    Please include any further details which are relevant to the design such as ground conditions, groundwater levels or other obstructions, any restrictions on the soakaway dimensions, nearby structures or any further information which might be relevant.

    File Attachments

    Also please attached any layout drawings, photos, planning documents or any other documents which might be relevant to the design.

    Related Spreadsheets from CivilWeb;

    Soakaway Design Spreadsheet

    This spreadsheet calculates the requirements for a soakaway system and assists the user to design a suitable system.

    Attenuation Design Spreadsheet

    This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system.

    Runoff Calculator Spreadsheet

    This spreadsheet calculates the design runoff flow for a site in accordance with the a number of different methods including the Wallingford Procedure.

    Full Drainage Design Suite

    Full drainage design suite (Save £40) including 7 spreadsheets;

    • Colebrook White Pipe Design
    • Manning Pipe Design
    • Manning Open Channel Design
    • Linear Drainage Design
    • Runoff Calculator
    • Attenuation Design
    • Soakaway Design