I have friends who are in this case. They worked in their flat first to save money, it is true.

But they grew tired of working in their apartment and started to rent a place as soon as they had sufficient funds (after 2 years).

Their problems were:

* Lack of separation between worklife and the rest.

* Living and working with the same persons. They are really good friends but it was still hard apparently.

* Having to dedicate a big part of your apartment to the work (desks but also hardware, documents, etc.).

They don't need to all work in the same apartment though, they can work in their own.

In their case, they lived in the same apartment.

Do you know the average size of a Parisian apartment?

The one's I have been in have all been about 400 square feet.

Most Parisian apartments are tiny. You can forget about owning a free standing house. If you can afford that, you don't need investors in your start up... Or you're not in Paris at all. Think Manhattan. Paris' overall population density is comparable to Manhattan and much higher than overall NYC density.

I'm not sure if the density is really comparable to Manhattan, as in "1977, a height regulation of 37-meters for buildings was put in place" - after Tour Montparnasse was finished [http://europe.newsweek.com/will-skyscrapers-ruin-paris-33068...]

Depending on the product you're building, the Skype/Slack communication may not be enough. If, for example, you're a game designer, artist and an engineer who're iterating together on a gameplay mechanic, it's certainly not enough. And apart from the work-related communication, you guys would need to have much more friendly chats and lunches together just to build the team together.