Losing a garage can often be detrimental to value regardless of what you convert it into. It may sound daft these days, when most of us don't use garages to keep our cars in, but they are still a big selling point for houses. Also, garage conversions can be difficult to do right from an aesthetic point of view. This could cause a problem too. On the flip side, if doing this will make a significant difference to your downstairs area, then it may be worthwhile. Talk to estate agents; they know what sells.