If you are planning to build by direct labour, the best advice I can give you is to have very clear designs and specifications from the start.
It is critical to have a good system of communicating with your labour exactly what you wish to achieve, it is very easy on an extension to make small mistakes that have a massive impact on the schedule and obviously costs as well.
Needless to say you need to be sure you have satisfied the local planning requirements and or exemptions. The work must meet the current Building Regulations, you may not be a builder, this will not be a reason or excuse to fail in the legal obligations you must meet as a builder.
Other than that, speak with you neighbours and outline how the process may impact on them.
Good luck.