The very first thing you should do before you even begin working on a home extension is make sure your plans make sense and are feasible. You need to be sure that the extension will add to the value of your home and not take away from it, so ask a local estate agent for their opinion.
Next, you’ll want to check that you don’t need planning permission for what you have in mind, look into building regulations and speak to your home insurance provider and neighbours about the plan – you may need to get a party wall agreement drawn up.
You can then reach out to architects and home extension contractors in your local area and begin getting quotes and designs, and setting budgets. There’s a lot more too it, but this is the basic process I follow every time I undertake home extensions in Enfield.