Learn German: Die Bausubstanz ist teils gepflegt, teils sanierungsbedürftig. - The building fabric is partly well-kept, partly in need of renovation.

It functions as a two-part conjunction meaning "partly... partly...". Here, it connects two contrasting adjectives (gepflegt and sanierungsbedürftig) to describe different parts of the same subject (die Bausubstanz). Note that you must use a comma to separate the two halves.

Could I use zum Teil ..., zum Teil ... instead?

Yes! Zum Teil ..., zum Teil ... is perfectly synonymous with teils ..., teils ... and works the exact same way grammatically. Using teils is just a bit shorter and sounds slightly more elegant in written German.

How is the word sanierungsbedürftig formed?

It is a compound adjective combining die Sanierung (the renovation) and the suffix -bedürftig (in need of), joined by a linking -s-. You can attach -bedürftig to many nouns to express that something requires that action, such as reparaturbedürftig (in need of repair) or erklärungsbedürftig (in need of explanation).

Why use Bausubstanz instead of just saying das Gebäude (the building)?

While das Gebäude refers to the building as a whole, die Bausubstanz specifically refers to the physical "fabric" or structural materials (like the walls, roof, and foundation). An architect or real estate agent uses this word to emphasize the actual physical condition of the structure.