Groundworks and Drainage for a Rear Extension: What's Involved

Groundworks and drainage are typically the first phase of any rear extension project and one of the most variable in cost. What lies beneath the surface — service runs, existing drainage, unexpected fill material, tree roots — determines a significant proportion of the overall project risk. Here is what is typically involved.

Site Setup and Temporary Works

Before excavation starts, the site must be set up:

• Welfare facilities (temporary toilet, wash facilities) for the construction team
• Skip or waste removal provision
• Scaffolding where required for external wall construction
• Protection of existing structures, garden areas, and services during excavation

Utility and Service Identification

Before any excavation, the position of existing underground services — gas, water, electric, telecoms, and drainage — must be identified. A utility survey (CAT scan and sometimes a GPR survey for deep services) reduces the risk of striking a service during excavation. If a gas or electrical service is struck during groundworks, the programme stops until it is made safe — at significant cost.

Excavation

Foundation trenches are excavated to the depth specified by the structural engineer. In London clay, this is typically 1.0–1.5m for standard residential extensions without tree root complications, and deeper where large trees are nearby.

The excavated material (London clay or made ground) must be removed from site. In urban London, skips and tipper trucks accessing narrow rear gardens add time and cost. Narrow-access excavators (tracked mini-diggers) are commonly required.

Foundation Concrete

Concrete is poured into the prepared trenches and allowed to cure before wall construction begins. Concrete mix specification and depth of pour are determined by the structural engineer's design.

Drainage Works

A rear extension typically involves:

• New drainage connections: The extension's floor drainage, WC, sink, or washing machine connections (if in the extension) must connect to the existing drainage system. New pipework runs are laid in the ground to the existing soil and drain connections.
• Retention of existing drainage: Where the extension overbuilds existing drainage runs, these must be diverted, relined, or — in some cases — built over with a build-over agreement from the relevant water authority.
• Sewer Build-Over: If the extension is built over or close to a public sewer, a Build-Over Agreement must be obtained from Thames Water (the relevant water company for most of London) before work starts. This requires a drain survey, application, and approval.

External Levels and Hard Standing

Where the extension connects to the garden, external levels, paving, and drainage of the external area are addressed after the extension is built. Changes to the patio or garden levels must not compromise the damp proof course of the extension or existing house.

RCB Design & Build

RCB Design & Build manages groundworks, drainage surveys, and build-over applications as part of the extension principal contractor service — reducing the risk of groundworks surprises derailing programme and budget.