If your subfloor is already hardwood but you want to improve the look of your home with new material it is possible to lay engineered hardwood over the existing floor with proper preparation and traditional installation techniques.
How to install solid hardwood floor over wood.
To learn how to do this watch prepping plywood subfloors.
They are available in 3 4 inch thickness as well as low profile styles that are 5 16 to 5 8 inch thick which are a good or installing over existing flooring.
When installing new wood flooring parallel to an existing solid nail down floor add a minimum of 3 8 plywood underlayment over the existing floor to increase stability.
For instance solid hardwood flooring is typically nailed down during installation while tongue and groove engineered flooring is usually clicked together or completely glued down.
Solid wood floors should not be installed in basements or below grade.
Like unfinished hardwood floors the prefinished types typically have tongue and groove edges on all sides.