So this is a theoretical question about next year, because you will be applying for a September 2018 place, is that right? And there is a real possibility that you may move house between the date that the application is put in and the date when your child starts school?
Depending on the area you live in, and the history of admissions being 'played' for that school, house moves both before and around school admissions time may well be investigated - without you knowing, in most cases, since the evidence trail will be e.g. council tax bills, land registry etc.
What is and is not allowed is probably easiest to illustrate using examples.
Family A owns a house which has always been just outside catchment. So, around application time, they rent it out and rent another property just a short distance away, but now within catchment. They apply from the rented house / flat, but have always intended to return to the permanent address either after allocation day or once school has started. This is not allowed, and is increasingly well-detected.
Family B are like family A, but have actually always owned a small property within the ctachment. They 'flip' their main address, carefully transferring e.g. doctor's contact address, primary school contact address, to the smaller property at the appropriate time. This is not allowed either.
Family C are moving into the area from a town 30+ miles away due to a change in employment. They do own a house in their old town, but rent it out and rent a new property in the catchment area. This IS allowed, because it is a genuine move, not just for admissions purposes.
Family D currently live in the catchment area, and have done since their child was 1. However, with the arrival of their second child, they are looking to move to a larger property, a little further away. At the point of application, they are living in the first house, but find and buy the second house within the months between application and starting school, selling the first. This IS allowed in general, though there would be a grey area if e.g. the old property had already been sold, and the new property available to be moved into, but the stay in the old property 'artificially prolonged' in order to gain advantage in admissions.
Does that help?