Can I take pictures of the outside of someone's house without consent? What if nobody owns the house and it is on sale?
Disclaimer: I am not a lawyer. The information below is general in nature and should not be taken as specific legal advice. If you have a specific legal question you should consult an attorney practicing in your jurisdiction that is familiar with the law and case history regarding the issues you wish to address.
The following general information is based on the assumption you are located in the United States or a country with similar laws regarding where and when making photographs of others' property is allowed.
In most countries that follow the Berne convention with regard to copyright laws, which includes the United States, the laws regarding taking photos of private property that can be seen from public areas is usually also very similar. The following applies to the U.S. but many other countries are similar.
In the United States, there is no expectation of privacy when in a public space. The courts, including the United States Supreme Court, have ruled on numerous occasions that even private property that is visible from public spaces is not protected by privacy laws. As long as one is standing in a location that can be legally accessed by any member of the general public, one is free to photograph whatever one can see.
There are some qualifications, though. If a property has a tall fence around it that prevents a pedestrian from seeing over the fence, it's not okay to use a ladder or a bucket truck to rise above the fence and take photos of what is inside the fence. With the advent of drone cameras, this legal doctrine had been expanded to include such remotely controlled devices.
I think officially it's not needed, although it depends per country (I'm from the Netherlands).
It might also depend if they are published or are used on public internet sites (and the context in which they are published).
Some time ago my parents had their 50 year wedding anniversary, and I made pictures of the houses they lived. I rang the door bell to ask explaining the situation; nobody gave a problem. For those who were not home I couldn't ask. But these pictures where only show to like 100 people once.
I would say: ask if you can, if not, too bad, but on the other hand they might even let you on their property to make a nicer picture. And of course if you are a decent photographer, you can give them the (post processed?) picture and the link to the website or copy of the medium in case it is published.
Google street view and some other navigation companies also make photos of houses without asking (as long as they are themselves on public property).