While Matt is correct, I'd only worry about these issues if a) you were considering that the changes would make the property as a whole more valuable, or b) the work involved was extensive and costly.
Reason being, if there was no value increase to the property and the work was as simple as a bit of insulation and plasterboard with some extra sockets and lighting it may be cheaper to do it and return it to a garage upon sale than go to the extra expense of involving a BCO.
If a BCO signs the work off, you will possibly need to comply with some regulations that you would otherwise not worry about. I'm not talking about compromising on safety, but you might be happy (read ; I would) with a lower spec space than may be required to satisfy regs in order to bring it into part of your house. Just my 2c.
I'd be happy to help discuss the options and cost implications if you wanted any help, we do these from time to time. I'm not touting for work, just happy to help answer questions if you have any in this area.