If you intend to drastically alter the allowed design, you must submit a new planning application. Moves are being made to reinstate it through a change in the legislation, so always check with your council's planning department to see whether modest alterations may be accepted without the rigmarole of a new planning application. Generally, these can be done at no cost to you; sometimes there is a small charge for admin time. The main reason people want to change their house plan is because they find that it isn't working for their family anymore. Sometimes this is because of changes in their lifestyle - maybe they don't need as big a house now that they have a child - others may want a more modern look. Whatever the reason, if you're not completely satisfied with your house plan, then you should consider changing it. You will need to draw up new plans for any major changes you wish to make. You should also get professional advice from an architect or planner before you start work on your project. This will help you avoid making mistakes that could end up costing you lots of money. Don't forget to keep copies of all the drawings and documents related to your project - this includes contracts, statements of compliance, and permits - just in case something goes wrong with your project later on. It is important to note that once you've received your planning permission, it can only be changed by applying for further planning permission.