Building on Undeveloped Spaces

When performing the Build action in a location with multiple undeveloped industry spaces, you must follow these rules (from page 9):

• "If possible, place it on a space displaying only that industry’s icon."
• "Otherwise, place it on a space displaying that industry’s icon with another industry’s icon."
• "If no undeveloped spaces display your industry’s icon, you may not place it in that location. Exception: (see 'Overbuilding')"

This means:

• You are required to prioritize single-industry spaces (spaces displaying only your industry’s icon) before using dual-industry spaces (spaces displaying your industry’s icon and another industry’s icon).

Overbuilding

The rules for Overbuilding (from page 10) do not mention any requirement to prioritize single-industry spaces over dual-industry spaces. Overbuilding is described as:

• "Sometimes you are allowed to replace an already placed Industry tile with a higher-level tile of the same industry type (you must still pay the necessary build costs)."
• There are specific rules for overbuilding your own tiles and opponents' tiles, but no mention of prioritizing single-industry spaces.

Summary

• When building on undeveloped spaces, you must prioritize single-industry spaces before dual-industry spaces.
• This prioritization rule does not apply to overbuilding.

Source: page 9 (Build action rules), page 10 (Overbuilding rules).

Source: Core Rulebook (pp. 10, 12, 8, 9)