Width of fireplace opening

[lfcjfc]

Is there a typical width required for a fireplace/chimney opening to take a "standard" hearth and surround. In my new build, one fireplace opening (at block stage) is about 25", the second is considerably less - I'm worried that I wont be able to find anything to fit the second when the time comes. Probably not too late to rectify - should I take action?

 

[Carpenter]

A standard fire place recess is 18" or 450mm wide- the length of a standard concrete block plus 1 mortar joint. The next size up is 20" or 500mm wide. However note than blocklayers will often leave the recess a little oversize, allowing you the option to go for a bigger fireback and surround. As long as your recess is a minimum of 18" wide you have nothing to worry about.