Haus Martin is very good but it is NOT a b and b as stated. lt is a kind of pension, and has many single rooms, all with wash basin, hot and cold water, microwave, fridge and tv.
The shower and loo are at the end of the corridor and are communal, but often there are just a few singles or a couple staying there so it`s not bad.
The price is very reasonable, around 24 euros pn, doubles obviously more. l only rented a single room, but l know one of the doubles on my landing didn`t have a washbasin in the room. l don`t know if ALL of them are the same but it is a distinct drawback in my opinion, though some may not mind.
The owner, Herr Krapf, is a very nice man, if you arrive later than around 7.30 you have to pick the key up at the pizzaria on the corner, almost opposite Haus Martin, and pay Herr Krapf the following day.
I have stayed there many times. Darmstadt is a lovely city, and Haus Martin is in a little and very peaceful side street very close to the Herrengarten and a church (Kirche) so if you come by taxi from Frankfurt Airport, make sure the taxi driver knows Darmstadt well. Not all do, ask in German " Kennen Sie Darmstadt gut, ( goot) ? and arrange a price beforehand !!
Although no breakfast is included there is a small supermarket neaby, called a supermarkt in german, and by walking through the lovely park, the herrngarten, you will get to the town centre, about a 15 minute walk, which has a Tegut supermarket, pronounced taygoot, also Karstadt dept store, in the Luisen Centre has a small supermarket in the basement plus a nice cafe/restaurant on the 2nd floor ( Le Buffet) and there is also an Aldi in the town centre, right next to Saturn.
ps there is no lift in Haus Martin, but as it`s only a couple of floors above the ground floor most people should manage it ok unless they are very, old or very handicapped.