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 and you should aim to secure planning permission before you enter into a contract with any building firms in case your planning application is rejected. 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.).