Building a Townhouse – additional charges/expenses?

I am new to property investing and I was wondering whether people could provide some advice about building a townhouse.

My partner currently owns a block of land that has a house on it, but has council approval to build a roughly 100sq townhouse on it. We are looking at this possibility and estimate it will cost up to $200,000 for construction of a townhouse that fits on this size block.

I can appreciate that prices vary depending on location, but could anyone provide a list of so called “hidden” expenses that we may encounter or things that we may not ordinarily be aware of?

I’d also appreciate any feedback/advice/hints regarding the construction of townhouses in general.

Thanks