Underpinning is the process of strengthening and deepening existing foundations. It's almost always necessary when converting a cellar into a habitable basement or excavating a new basement beneath an existing house.
Why Underpin?
Most London houses have shallow foundations — sometimes only 600mm deep. To create a basement with adequate headroom (typically 2.4m floor to ceiling), you need to dig well below the existing foundation level. Underpinning extends those foundations down to the new, lower level.
Common Methods
- Mass concrete underpinning — traditional, done in short sections ('pins'), suitable for most residential projects
- Mini-piled underpinning — uses small-diameter piles, ideal for restricted access or poor ground
- Beam and base underpinning — a reinforced concrete beam spans between new pad foundations
The Sequence Matters
Underpinning must be done in a carefully planned sequence to avoid destabilising the building. Your structural engineer designs the pin sequence, sizes, and reinforcement — and specifies the temporary propping needed at each stage.
Need help with your project?
Our engineers are ready to discuss your requirements.
