Okay, in the Execution phase the Terrans use a mobilize order and attack an area with a Zerg base on it. The Terrans kill ALL the Zerg defenders.

However destroying bases does not occur until Regrouping. AND-- There are still several orders left in the stack.

The next one up is a Zerg Build Order.

So, can the Zerg player build more units in this area. Even if enemy units are present? If so, does a battle commence?

Thnx

If there remains any area on that zerg planet that is empty, zerg may build there their units. They may not build in an area with their base, which is occupied by terrans (as you wrote).

Other solution could be using collateral damage, supposing that zergs would have defenders at their base area. In that case terrans would not have to wait (and survive there) till regrouping phase.

That's explained in rules book:

"To build a unit, a player pays its resource cost by harvesting
the resources indicated on the Faction Sheet below the
desired unit, and then places the unit on any friendly or
empty area on the active planet."

Cool - Murky buckets!

I think the question being asked here is: "Is the area in question a friendly or enemy area?" The simple answer is: neither...for now. It depends on whether or not the area can be retaken by your opponent before the Regrouping Phase. Until that time, neither you, nor your opponent can build units in that area. However, it now could become a race to see who can build units in any empty area(s) first.

The base itself is left untouched until the Regrouping Phase (unless Collateral Damage is triggered), which could theoretically give the opponent the opportunity to retake his base by mobilizing from another area or planet. It is also possible for the base to be retaken by building new units in an empty area on that planet and then use those to attack.

I have, in some situations, chosen to build new units in an empty area and then, in the same build order, opted to destroy my base in order to build one in that same area as my new units. Among other benefits, it allows me to keep some, if not all, of the resource cards from that planet, because I will still have a base present during the Regrouping Phase.