check with the local council about your specific block before committing otherwise we may give a general answer for a specific question that can be different depending on what you are aiming to achieve...why do you want two dwellings on the block?.. ask Council about maximum building footprint (and what's included) but do not use the term granny flat as the answer may not include you specific plans...or choosing an alternate or flexible option might suit you just as well but ask can you have separate buildings and maximum setbacks between building and all four boundaries etc

In WA we have the R Code system. This dictates the total allowed percentage for buildings and open space on the block. Each suburb has areas of different R Codes. It is not habitable space, but open space to building space ratios. Laundry would be a building.
As we don't know which state you are in, then as oklouise mentioned, you need to check with your council for the specific rules for your piece of land.