Agreed, its important to be realistic. With the hot tub, I made a detailed list of all expected expenses, plus margin for unexpected - I think I heard that projects usually cost about 20% more than the estimate. It ended up costing quite a bit more in practice, so I checked with people to see if they were okay with the upward adjustment, and then asked for a max that they were willing to spend so that I could continue working on it without continued asking at every point. We were able to make it happen for under the max.

Another trick with group houses is making payment optional/voluntary - sometimes projects can get blocked because not everyone wants to contribute equally, so its good to just ask people to chip in what its worth to them, or whatever it is that they want to chip in, and see if you can cover expenses in a way that works for everyone. There are a lot of different options.

As far as people not living up to expectations, that can definitely be an issue. I personally get around this by trying to automate as much as possible. Getting a housekeeper, and including expense of paying the housekeeper to take care of the hot tub was part of the expense estimate I gave that everyone agreed to.