Joining a New Foundation to an Existing Building

The masonry entry is direct about this: older buildings rarely carry a record of what was altered and when, so establishing what is carrying, what has already been removed and what is currently propping what is a survey exercise, and assumptions made from appearance are unreliable.

The same is true beneath the ground. What the existing foundation is, what it bears on and what it is carrying are established by investigation rather than assumed, which is why the first item in an extension foundation discussion is usually a trial hole rather than a drawing.

Founding level is chosen for the stratum and the influences around it. Matching the existing foundation therefore means understanding what it is founded on, not measuring how deep it happens to be, because two excavations to the same depth can arrive at very different formations.

Where the new work has to go deeper, for trees, for a different stratum or because the existing is shallow, the new foundation is founding lower than its neighbour. That is a step in the foundation line, and the wall above spans a transition between two different bearing conditions.

New construction applies a load the ground has not carried before, and the ground responds. The existing building applied its load a long time ago. Even where both are founded identically, the two halves of the finished house are at different points in the same process.

Differential movement between elements founded differently is the characteristic risk the library keeps naming, and it is what the junction detail exists to deal with rather than to conceal.

Two approaches exist and they are opposites. Tying the structures together makes them act as one and relies on the connection being able to do that; separating them accepts that they will move differently and provides a joint where the movement can occur.

The masonry entry is useful on why this is not a detail to improvise. Ties between new and existing masonry are what make separate elements act together, and where they are missing, elements that look joined behave as if they are not. A movement joint bridged by a tie or a bonded return is not a joint at all.

An extension usually opens the wall it joins. That wall is carrying floors and roof and is also being held in line by the floors and returning walls that meet it, and an opening interrupts both duties at once. Widening an opening lengthens what is being carried and shortens what is carrying it in the same operation.

Whether the existing foundation is being asked to carry a different load or the same load differently distributed is the question the entry puts to professionals, and beam-end reactions arriving at a pier are a concentration where a distributed load used to be.

Digging the new foundation means taking ground away next to the old one. The strip entry names later trenching alongside a strip as removing support the strip was relying on, and the drainage entry repeats it: a trench beside a foundation removes support, and a foundation bearing over a drain imposes load a drain cannot take.

That makes sequence part of the design rather than a contractor's choice, and it is why temporary support may be required before anything is opened.

The new floor's membrane, the new wall's damp-proof course and whatever exists in the old building all have to become one line at the junction. Where the existing course sits relative to the new internal floor and the new external ground level is set by the old building rather than by the new drawing, and that constraint is easier to meet before anything is cast.