The answer to each of your questions is "an opinion," and that's a fact.
Goodness is a vague term which will depend on individual circumstances, perceptions, biases, expectations, etc. If these variables were identical, everyone would live in the same place in identical dwellings, drive the same cars, eat the same food, etc.
It Depends. It is a fact if you have parameters that can be indicative of Goodness. In safety, for instance, a building can show good features and bad features - therefore more or less safe, this defines the goodness of the building. This is a fact not an opinion. If you ask people about the way they assess that particular building - their assessment will most certainly vary, reflecting some informed guesses about safety risk (objective dimension) plus preferences in terms of style and other building and location characteristics (Subjective dimension). Does it make sense?
Answered in context that considers two key features of your question:
Is the goodness of an environment
a matter of fact, or, of opinion?
You will first need to qualify/quantify what makes an environment good. For example an outdoor urban environment requires multiple infrastructures to work together well for the optimal functioning health and wellbeing (HWB) of mankind; therefore it is not only about the architecture of the buildings.
In that context there are scientifically reliable measures that may be employed to establish the effects of the environment on physiological responses that will provide objective quantifiable factual outcomes.
However, what cannot be underestimated and also needs consideration in order to come to a robust conclusion - is the opinions of the people who live in this environment, as they are experts on their surroundings and what happens in it and what affects them directly.
That said, what may be considered hearsay or may be interpreted as perception that is skewed by life experiences of that individual may also be examined more closely by going for a walk with the person in their environment and see/photograph what it is that they are referring to. While you are doing this you are recording the 'evidence behind the perception' - which then may become another reliable objective factual data source.
My short answer is that it is a fact rather than opinion. For example, a doctor based on his diagnose with a patient says that the patient's tumor is a cancer. This say is not just an opinion, but likely to be a fact, something that exists there. On the other hand, a group of school kids were asked their opinions about McChicken, and they may say "we all like McDonald". This is indeed an opinion expression rather than a fact.
I will address this issue via a full-day tutorial: Poster Goodness of Space As a Matter of Fact – Rather than Opinion ...