It depends on how you do it. If you convert your garage into livable space, that will typically increase the value of your house, as it'll now have more square footage. However, if you just use the garage for storage and don't change its living status, that might devalue your house a bit, as potential buyers might see it as not being able to be used for its intended purpose (parking cars).

It can, but it doesn't have to. There are a lot of factors that go into whether or not converting your garage will devalue your house. The most important one is deciding what you're going to use the garage for. If you convert it into a livable space, like an extra bedroom, bathroom, or home office, it's going to be more valuable and it won't devalue your house. But if you just use it for storage, then it might lower the value of your home because potential buyers might think you have too much storage space and they'll be less likely to want to buy your house.