Doubt any major builder will change an en suite to a wardrobe as they like to insist on sticking to the plan.
They will quote that they will need it to be the same as the plan if you pull out and they have to resell. Taylor Wimpey for example will not even remove an electric point (even though I pointed out a bed would cover one of two and could it be moved a few inches to the left - after arguments, they agreed to install a second for free rather than charge me £50)
Regarding the garage, this will depend on a lot of things. Local authority planning permissions (we cannot change our car port into a garage, TW even covenented that as it was part of obligations), willingness on part of builder, and how much you agree to be overcharged - and they will over charge you.