Most home extensions will require a solid foundation to be laid, with groundworks to prepare. Depending on the layout of your plot and extension plans, groundworks can be a large part of the build. Hard ground or sloping sites can make it more difficult. It is important to make sure that the ground is firstly cleared of any turf, plants, or shrubs. The area where the extension is to be built, along with an average of a metre of space around it, will be cleared and dug down to a level that is sufficient to allow the floor and supporting layer to be put down.

The pit for the foundation will then be dug around the edge of this area. Depths vary depending on the materials being used and the height of the extension. Specialist foundation methods like anchoring or piling may be used in the case of some tricky grounds like rock or soft clay.

The materials used for the foundation layers can differ depending on a number of factors such as the ground type and the type and size of the extension to be built. The pits may be filled almost to the top with concrete for very large extensions. This technique is known as a trench fill foundation and the walls are built on top of the concrete. However, smaller extensions do not usually require the pit to be filled, and in these cases, a deep strip foundation is most commonly used. Using this technique, the whole underground part of the foundation is built up with blocks aside from a small layer of cement on the bottom. Eventually, the materials are switched to brick and stone once the layers begin to reach the ground level.

Whichever method you use for laying the foundations for an extension, ground-level will eventually be reached, making the switch to the decorative layer. This is typically applied on the outside of the two walls. The foundations are usually made from two walls so that there is an insulating cavity between them that is carried on throughout the walls of the extension itself. Any decorative layers of brick or stone are usually only applied to the outside layer, with cheaper building blocks used on the inside layer since they will be out of sight.

Once the foundation walls have reached the height of the damp-proof-membrane, the foundation is considered to have ended and this marks the beginning of building the extension walls.