House extension foundations or footings
Most house extension foundations will normally be deeper than the original house and many homeowners
find this peculiar and strange. This is because the requirements for most modern day foundations are deeper than of
old. Even slightly suspicious ground conditions or a local tree will cause the Building Inspector to require deeper
foundations for the house extension.
Most Councils use the only bit of external guidance for foundations depths by using the
NHBC foundation tables. An alternative approach for the home owner is to have your own engineered designed
foundations for when the Councils tabulated approach becomes very expensive and possibly excessive.
Most house extension footings or foundations are usually 600mm wide x 800mm deep x 1M min. depth trench fill
foundations for most good to medium quality sub soil ground conditions. Most chalk soils are excellent sub soils
and even local trees do not excessively affect the foundations depth or design.
Should clay sub soils be evident then things are totally different and expensive heave protection and extra
depths will be required unless an engineered piled or pad and beam design of house extension foundation is
utilised. As a guide, most unknown costs or extra over costs for most house extension scheme occur during the
foundation and ground floor stages of construction.
This is because most homeowners will not invest in the extra cost of obtaining a soil test report (approx.
£1,500.00) prior to engaging their builder so certain base level assumptions have to be made by the Building
Designer that may deficient when the site ground conditions are exposed.
It is not reasonable for your builder to absorb these extra foundations costs in material
and labour. A good way to control the possible extra costs of your house extensions foundations or footings is to
agree with your builder that any additional work will be at cost price to the builder plus say 10% for profit and
attendance. This way is then fair to both sides in my opinion.