Some do not know this, but an existing residence can be expanded on most servers using the Residence plugin
People often tell me it cannot be done and its tricky, but it is possible.
These are the commands to do it.
/res select residence YourRes main
/res select expand 10
/res area replace YourRes main
Obviously you need to face the direction you want to expand prior to typing the expand command, and 10 can be changed to any number of block you want to expand it.
Typing /res select cost before the expand command and after will show you the difference in cost. It will always say the cost of the WHOLE res, but it will only charge you for the section you are adding, so you need before and after to calculate what will be charged.
You can also accomplish this using a selection tool. First select an area with a selection tool that includes the residence. Next type the area replace command.
/res area replace YourResName main
Hopefully, these commands will help reduce the cost of increasing your residence as well as make you more willing to invest in a smaller residence when you have intentions of expanding in the future.