What happens if there is not material in the lower denture for the metal housings?

If the denture is not too old and the denture teeth are in good condition, it might be possible to place a new base on the existing denture at the same time the housings are captured.  Although there is a charge to place a new base on the denture, the cost would be less than a new complete denture.  This assumes that you are happy with the shape, size and color of the teeth on the existing denture.

The average thickness of the denture base should be enough. If there is not then a new denture is recommended.

Your denture has to have a minimal thickness to accommodate the height of the abutment (that is attached to the implant) and the housing.  If there is not enough height, the denture will break at that point where the housing is located, or the housing may protrude out of the tooth side of the denture.  If this is an old denture, then have your dentist make a new one at the proper thickness.  If the denture is new, then you have two choices:  one, change the abutments to a lower height, or two, make a new denture.