Basements, retaining walls and underpinning in London
Basement impact assessments, structural method statements, retaining walls and underpinning sequences for new and extended basements.
About these services
Digging below an existing house is one of the most demanding structural projects in London. This group covers the reports councils ask for at planning stage, such as Basement Impact Assessments, ground movement predictions and groundwater screening, along with the structural design that follows: underpinning sequences, retaining walls and method statements. It also includes condition assessments for existing garden and boundary retaining walls, which are common on sloping streets.
Homeowners planning a new basement or lowering an existing cellar are the main clients, along with their architects and party wall surveyors. Small developers adding lower-ground flats use the same reports. Neighbours and their surveyors sometimes commission a ground movement review to understand the risk to their own property. Owners of older retaining walls, particularly where a garden sits higher than the house or a driveway cuts into a slope, use our condition assessments.
Several inner London boroughs, including Camden, Westminster and Kensington and Chelsea, have specific basement policies, and Building Control and party wall surveyors expect clear design information too. London Clay, perched water and shallow neighbouring footings all shape the design. We start with the ground and the neighbours, then design the underpinning and retaining walls to suit, with each section dug and cast in a sequence that keeps the existing house supported.
Services in this group
- Basement Impact Assessment (BIA)GCE-BA-001from £2,000
- Basement structural method statementGCE-BA-002from £750
- Ground movement and damage assessmentGCE-BA-003from £1,500
- Groundwater and hydrology screeningGCE-BA-004from £750
- Underpinning design and sequenceGCE-SD-024from £1,000
- Retaining wall designGCE-SD-011from £600
- Retaining wall condition assessmentGCE-IN-015from £500
Which basement reports does your project need?
If you are applying for planning permission for a new basement, your council may require a Basement Impact Assessment, which looks at ground stability, groundwater, drainage and neighbouring buildings together. Some councils also ask for a separate ground movement and damage assessment predicting the likely damage category for adjoining houses. Your council's validation checklist will confirm exactly what is needed for your borough and your site.
Once planning is in hand, the underpinning design and sequence gives your builder the dimensions, reinforcement and the order in which each section is dug and cast. A basement structural method statement is often needed for party wall purposes as well. For an existing wall holding back a garden, start with a retaining wall condition assessment before deciding whether repair, strengthening or a full rebuild is required.
Projects using these services
Frequently asked questions
What does a Basement Impact Assessment include?
It normally screens and then assesses the effects of a basement on ground stability, groundwater and surface water, and considers the impact on neighbouring buildings. Depending on the findings, it may draw on a ground investigation, a ground movement assessment and a drainage review. The exact scope depends on your council's policy and the site, and the planning officer will confirm what they expect.
Is underpinning the same as building a basement?
Not quite. Underpinning means deepening or strengthening existing foundations by casting new concrete below them in short sections. It is used to stabilise a house affected by subsidence, and also as the method for forming basement walls under an existing building. In a basement, the underpins are usually reinforced and designed as retaining walls that hold back the surrounding ground.
My garden retaining wall is leaning. Is it dangerous?
It may be. Leaning, bulging, wide cracks and soil washing through joints are all signs that a retaining wall is struggling, often because of poor drainage behind it or tree roots. If the wall is high or next to a path, keep people away from it and arrange an inspection soon. We assess its condition and advise whether to repair, strengthen or rebuild.
Will a basement affect my neighbours' houses?
Any deep excavation causes some ground movement. The aim of good design and careful sequencing is to keep it small, so that any damage to neighbours is limited to fine cracks at most. A ground movement assessment predicts this before work starts, and a schedule of condition records their property beforehand. Party wall procedures also apply, giving neighbours a formal say in how the work is done.