Germany: Professional accommodation for one month?

You have a couple of options:

• Look at a meta-search engine such as Trivago.de -- They also have some non-hotels in their database
• Look on AirBNB. While the legality is sometimes questionable in Germany, that's not the problem of the renter.
• Look for vacation rentals. While Bochum is not exactly a place for vacation, there may be some nearby. Search engines such as Wimdu.de could be a starting point.
• Perhaps you may have a look at wg-gesucht.de -- This website is often used by students for offering rooms in flat-share communities for sub-letting while they are away (e.g., on an internship). Knowledge of German is highly useful there, though, and a month of rental may be too short for many offers.

The classical flat rental market is not what you want. Few landlords will be willing to offer a rental contract for this short time period, and the default style of renting a place in Germany is without furniture. Furnished apartments are quite unusual.

I do not have any connection to any of the mentioned websites (apart from having been a customer).

What you are looking for is called corporate housing. They go by other names such as "serviced apartments". Basically it is short term accommodation, larger than a hotel room and with a kitchen and other facilities. It's specifically designed for people in your situation, where the stay is long enough to make being in a hotel troublesome but too short to be in the normal rental market, and where an employer is paying the costs. They are significantly larger than a hotel room, usually apartment sized or townhouse sized. They always come not just fully furnished but with everything you need to live a normal life, and often come serviced - i.e. they get cleaned on a regular basis. The price is usually less than a hotel but more than an apartment rental (though probably less than an apartment rental plus furniture etc.)

Places like this exist in most countries. Here is a page talking about them specifically in Germany.

If you speak at least a bit of german, or can get someone who does to help you, you can check the following ressources:

• the major nationwide real estate web sites such as immobilienscout24.de, immowelt.de, immonet.de
• the housing market sections of local newspapers such as ruhrnachrichten.de
• ebay Kleinanzeigen, the german equivalent to craigslist
• wg-gesucht.de specialises on appartment-sharing and is even available in english (though the ads themselves might be available only in german)

Helpful keywords/categories are "möbliert" (ready-furnished) and especially "Wohnen auf Zeit" (living for a limited time, short-term rental). The latter almost always implies the former.

Furthermore, you can try directly typing "Wohnen auf Zeit" and "Bochum" into google (or whatever other general-purpose search engine you prefer), or even only "Wohnen auf Zeit", and enter "Bochum" as the next step into one of the web sites specialized on short-term rentals turned up by the search.