House Extension Underpinning - When is it required?

Most house extensions require some form of underpinning when completing a new addition to the home. However, this is usually very localised and quite normal. This is usually due to the requirement that the new foundations for the house extension will be deeper than that of the existing property.  This means that in most cases the Building Inspector will require the new concrete foundation to be formed partly under the existing house footings at the abutment point.

Underpinning any other part of the existing property for a home extension is very rare.  The situations when this can happen are as follows:-

FIRST FLOOR HOUSE EXTENSION WORK OVER THE EXISTING STRUCTURE - Where increased loads are being placed onto the existing structures, the existing foundations will need to be exposed and checked for adequacy by the site visiting building inspector.  If there is any doubt that they will be adequate the Building Inspector usually wants some form of underpinning. 

However, your first port of call should this happen should be to a local Structural Engineer as they can often assess the bearing pressure of the exposed sub soil and calculate the loads onto the existing area of foundations.  Should they be able  to prove by calculation that the existing foundations are adequate for the increased loads then the Building Inspector will retire happy and allow the builder to carry on.

LOCALISED BEAMS BEARING ONTO EXISTING WALLS OR PARTITIONS - Sometimes many internal walls of existing houses were only built up off a thickened concrete floor beam rather than a formal deeper foundation.  Underpinning the house extension in this situation is fairly typical.

As this underpinning work for a house extension can add significant costs to a project, it would be unfair to expect your house extension builder to absorb these costs.  In most cases they have only allowed for the normal 1M deep foundations to the new extension work without extra underpinning to the existing property.

In some situations where the house extension is being built on really poor quality ground (made up or highly shrinkable clay for example), the new extension needs to be built up off piled foundations which could reduce the extent of any knock on underpinning to the existing property.

Some house extensions fail after a period of time due to the wrong type of foundations being installed for the sub-soil on site.  This is where the house extension itself needs underpinning.  Fortunately this situation is pretty rare as all foundations are usually inspected and approved by the site visiting building inspector.