Just a quick review for Apartments Duval in Untermainkai in Frankfurt. (No individual website, but listed on various booking sites.)
I was slightly dubious about this place, having found it while looking for a cheap rate on a booking site. But reviews were positive and this is going to be a positive one as well.
Duval is a set of bedsitting rooms within an early-twentieth-century jugendstil tenement building on the north bank of the Main, ten to fifteen minutes walk from the Hauptbahnhof and almost opposite the Staedel museum across the river. Some of the building is occupied by permanent residents, so be considerate. The cost is circa 60-70 euros per night.
You will need to arrange an arrival time with Herr Antic who owns the rooms, but I arrived after nine pm and he was perfectly happy. Payment is required on arrival, but he does take credit cards, which can't always be relied upon with non-chain German accomodation.
There is a small and slightly jolty, but reliable, lift. The building is not wheelchair-accessible, however, as there are two steps from the street into the building and five or six from the entrance hall up to the ground-floor lift landing.
Each room has an ensuite bathroom, and a food-preparation area with a fridge and a two-saucepan hob. You get a loaf of bread, butter, jam, coffee and tea on arrival. The rooms are cleaned daily and a small bottle each of orange juice and mineral water are delivered.
The rooms and the furniture aren't especially luxuriously decorated but are comfortable.
There is only one warning/disadvantage: the slightly more expensive rooms with river view do have a spectacular view of the Main and the Museumsufer. Unfortunately this comes with some traffic noise, despite double glazing, and the south-facing rooms can get very hot in sunny weather.
Other than that, definitely recommended.