While everything is conceivable in a self-build home project, annexes are limited to one storey and are considered supplementary housing to the main house. They can be used for a variety of purposes such as a garage, study or even a second bedroom. Annexes are usually built on separate levels or sites but they could also be under one roof with some internal access. They should not affect the rating of the building if it meets all relevant regulations but you would need to check this with the relevant authorities before you started construction. Self-builders tend to follow a standard procedure when constructing annexes. The first thing you will need to decide is what type of annexe you want to construct. There are two main types of annexe: off-site and on-site. Off-site means that the contractor builds the annex on a separate site away from the main house and then transports it to its final position. This is often more expensive than on-site construction because there is no use of land close to the main house where builders can save money by using pre-fabricated components or employing efficient construction techniques. On-site construction means that the contractor builds the annex directly onto the main house structure.