Building Regulations do not generally apply. However, you will need to make sure that any alterations do not make access to the dwelling any less satisfactory than it was before.
So, for example, changing levels to introduce steps where none existed before would be a contravention of the regulations.
If you are making a new access into the garden across the footpath you will need to obtain permission from the local council to drop the kerbs and the pavement may need strengthening. This is to protect any services buried in the ground such as water pipes.