It may not make much difference or it could be considerable but thicker or double glass in a bigger frame would be more expensive to maintain and repair. At the very least, without a by-law, the Owners Corps should have a say in the who is doing the installation and the materials they are using. It doesn’t sound like this owner is going to be a problem – but when the person he sells to, or the one after that demands expensive repairs or maintenance, whoever is running the EC then will wish there had been a bit more care taken with this agreement now. This about a letter of a greement and then a by-law for the future m(your next AGM, for instance) that says anyone who alters common property with or without Owners Corp permission will be liable for its repair and maintenance, as will any subsequent owners of the lot..