Before anything is dug
A basement project starts with information: a measured survey, a structural appraisal of the existing building, a ground investigation and, in most London boroughs, a basement impact assessment. Party wall matters are dealt with in parallel; awards typically fix the working hours, the monitoring regime and the condition schedules for adjoining properties before work begins. None of this is contractor work, but all of it shapes the programme, and a contractor who prices a basement before it exists is guessing.
Temporary works come first
Every stage of a basement is held up by something temporary. Needling, propping schemes, raking shores and edge protection are designed by a temporary works designer, installed to that design, inspected, and only then loaded. The single most common cause of trouble on residential basement sites is a sequence varied on site because it was inconvenient — a bay opened out of order, a prop struck early, a pin left open overnight.
The underpinning sequence
Traditional mass concrete underpinning proceeds in bays, usually no more than a metre wide, in a hit-and-miss sequence so that no more than a defined proportion of the wall is unsupported at any time. Each bay is excavated, cast against the retained ground, and dry-packed to the underside of the existing footing before the adjacent bay is opened. Dry-packing is the step that transfers the load; skimped, it produces settlement that no amount of finishing will hide.
Getting the spoil out
On a terraced London site, muck-away is often the constraint that sets the programme. Conveyors, mini-excavators and skip rotations have to be planned against permitted hours, parking suspensions and the width of the street. It is worth asking a contractor how they intend to remove the spoil before asking how they intend to dig it.
Waterproofing
Waterproofing is designed by a specialist, generally as a combined system: an external or structurally integral barrier, a drained cavity membrane internally, and a sump and pump with alarm and maintainable access. British Standard 8102 sets out the grades of protection. The details that fail are rarely the flat areas — they are service penetrations, construction joints and the junction between old and new.
Monitoring
On sensitive sites, movement and vibration monitoring runs through the works, with trigger levels agreed in advance and readings reported to the design team. Monitoring is not a formality: it is the mechanism by which a sequence is stopped and reviewed before damage occurs. Eternal implemented and coordinated monitoring of this kind throughout the below-ground works at Caithness Road in Brook Green, where a neighbouring property incorporated a substantial three-storey glass structure.