If there is not already detailed planning permission for your home for the plot of land you are buying, you will need to secure this before you can start building as the planning authority (your local council or national park authority) will need to be satisfied that your plans for your home are in line with the Council’s Development Plan(s) and that your home’s design will be acceptable in terms of its impact on the surrounding homes and landscape. Don’t pay for land before it has planning permission- don’t sign a contract or include a clause that it will be void if the property fails to get permission.
You should try to get planning permission as early as possible, as the process can take some time. Try to speak to a planning officer at the relevant council before submitting your application as they should be able to let you know if there are any issues you should be aware of or any restrictions they might impose (for example they might restrict the size or height of the home, its distance from a road or the plot boundary, etc.).
The council should also be able to help if you need advice or guidance on completing a planning application. There will be a fee for submitting an application. The planning authority will notify any neighbours, but if your plot has no immediate neighbours, there is also currently an additional charge to advertise your planning application in the local newspaper(s) so that any other residents who may wish to comment have an opportunity to do so.