Or HaChaim on Terumah 27:1

English: ועשית את המזבח , "You shall make the altar, etc." The Torah speaks of המזבח , instead of saying merely מזבח . Perhaps this is because the Torah refers here to what has already been mentioned in 25,9 when the Torah described that G'd had shown Moshe a blueprint of both the Tabernacle and its furnishings including the altar. When adding that the altar was to be made of acacia wood, the Torah elaborated that whereas G'd had shown Moshe a picture of the completed altar, i.e. a structure covered with copper, He now revealed that the altar was not to be of solid copper but of acacia wood which would be overlaid with copper.