Yes, you need planning permission to do nearly any kind of remodeling in your home.
In the case of a garage or basement conversion, you need to conform to building regulations, fire safety, and local zoning codes. A lot of people illegally convert garages into living spaces without complying with the local building code.
Not only is this extraordinarily dangerous for anyone living in an illegal unit, but it’s also dangerous for the homeowner because it leaves them open to lawsuits. Imagine you are renting an illegally converted garage to someone and they start a fire.
Not only are you 100% liable for the damage, but if they injure themselves you are on the hook for their medical bills and leave yourself open to a serious lawsuit.