The clearance requirement varies
In some circumstance, 24 inches or more is the recommended clearance. However, the manufactures installation may say that less space is required, i.e. 16 or 18 inches. Reviewing the manufactures installation recommendations and a check with the local building depart can give a more definitive number.
When the clearance is low, tall pots may not fit well or if you want to stir a pot or add more ingredients, then it may be difficult. As you can see, this is a safety concern at times and an ease of use issue.
A number of Manufactures refer to 15 or 16 inch clearance.
If there is wood trim or combustionable materials above the stove, then having a minimum of 30 inches of clearance is generally recommended. When the clearance are low, there is often a concern about heat and fire safety. Metal hoods and micro waves can be installed lower than 30 inches.
Who to consult? Kitchen re-modelers, appliance technicians and qualified handymen.
Home owners can often change out appliances or move hoods and microwaves. Likewise, Kitchen re-modelers, appliance technicians and qualified handymen. At times cabinet makers and installers can handle these type of tasks.