If your house has an attached garage or a previous single-storey extension, you could build a new extension on top of it. In other words, you are 'building over' the existing structure, hence the name.
Some older garages have single-skin brickwork unable to carry the load of another building on top. If this is the case, more structural work would be required to strengthen the original walls before a new extension can be built on top of the garage.
This type of extension project is great if you want to add an extra bedroom, office, study or a playroom for the children.