Floor preparation is extremely important when having a new floor covering on it. The installer would need to test your floor for dampness, blemishes and repair work before any preparation can go ahead. If you have a wooden subfloor and having a vinyl, luxury vinyl or laminate floor it is extremely important that the floor is smooth and free from any blemishes and would need to have a plywood over the top of it to be done in the correct way. If you have a concrete floor you would need to make sure it also is smooth and blemish free for your floor coverings. This would be carried out with a self smoothing compound spread by the qualified installer. But as concrete floors are not as simple as wooden floors by having a ply placed over the installer would need to know if the floor is standard concrete or if it is gypsum based or asphalt which can be more complicated to deal with, they all need their own type of smoothing compound over the top of them along with a suitable floor primer, if this is not done the correct way the floor can fail and lead to unnecessary expense.