Extension foundations meeting an existing building: the junction problems a clean elevation won't show
Where new foundations meet an existing building is one of the most common sources of extension defects — differential movement, damp-proof course continuity, and matching a modern foundation to one built to an unknown historic standard.
An extension's structural drawings will show the new foundation in careful detail and the existing building as a simple hatched outline, as if the two were unrelated. They aren't. The junction between a brand-new foundation and a decades- or centuries-old one is where a disproportionate share of extension defects actually originate, and it rarely gets the same scrutiny as either foundation on its own.
Two foundations that were never designed together
The extension's foundation is new, designed to current standards against a known ground investigation. The existing house's foundation is whatever was built when the house went up, often undocumented and untested. The junction between them is where two structures with genuinely different stiffness and depth meet, and that mismatch — not poor workmanship — is the underlying cause of most extension-related cracking.
Differential settlement, and why a movement joint isn't optional
New foundations, especially on clay or made ground, will settle by a small but real amount as the new load beds in — at a different rate to a long-settled existing structure that stopped moving decades ago. A properly detailed movement joint between old and new, carried through the full height of the building including the roof, is what accommodates this without cracking. Omitting it, or detailing it as a cosmetic joint that doesn't actually allow movement, is a common and expensive mistake that only shows up after the extension is finished.
Matching foundation depth, not just foundation type
A new foundation shallower than the existing one can undermine it during excavation. One deeper than necessary simply costs more than it needs to. The existing foundation's actual depth needs establishing — by trial pit, not assumption — before the new one is designed, and building control will expect to see that reasoning explicitly, not just a generic depth applied because it's standard practice.
Damp-proof course continuity
The new damp-proof course needs to tie into the existing one at a consistent height. Where the existing DPC is bridged, has failed, or — in some older properties — simply doesn't exist, the junction needs a specific detail rather than an assumption that the new DPC alone will protect the old wall from rising damp at the join.
Structural openings into the existing wall
Where the extension creates a new opening into the existing house — removing part of an external wall to connect the spaces — the lintel and its bearing need designing against the existing wall's actual construction, established by inspection rather than assumed from the house's age or general appearance.
Why this belongs in a buildability review, not just a structural calculation
A structural engineer will size the new foundation and the new lintel correctly in isolation. Whether the two systems have actually been reconciled at the junction, and whether the construction sequence protects the existing structure during excavation, is a buildability question that sits alongside the structural design rather than automatically inside it.