That depends where you are buying and what size your boat is. Here in Europe some of us have converted former freight-barges, so that allows plenty of room for personal effects etc.. It is not uncommon to see cars being carried on their upperdeck!
As an example, ours is 93 x 16 feet which gives us about 1500 sq ft of accommodation space.
Into this we have built a workshop with rack storage, paint locker and workbench; a lobby, where we keep the barbeque, 'garden' furniture, and bikes; in addition there are racks for trunks and suitcases etc. - The office (think 'den') also doubles as a library. The laundry not only holds the washing machine and tumble drier but also the chest-freezer. The logs for the solid fuel stove are stored in the cable locker which takes about three cubic yards of wood (although our primary heating is a ten-radiator central heating system).
Having said all of that Virunga is also a cruising barge and we travel from country to country across the continent of Europe (no borders - no airport queues - no packing of suitcases)!
In the summer we follow the rural life (mainly off the tourist track) usually rubbing shoulders and socailising with, predominantly: American; British; Canadian; Australian, and New Zealand barge owners!
In winter we like to berth in city centres; as an example, last year we spent the winter in the centre of Mullhouse (Alsace, France) which is only 30 mins from Strasbourg (France), 20 minutes from Basle (Switzerand), and about 45 minutes from Frieberg (Germany).
Then in the summer we headed north up the river Rhine, and then onto the rivers Moselle and Meuse through France. We then travelled the length of Belgium to enter Holland at Maastricht. After taking the boat out of the water for her scheduled, 7-year routine inspection and survey at Hasselt we then came back south to a winter berth about 30 mins from central Amsterdam.
If you'd like to see an example of a European cruising houseboat take a look at some of our photos at: flickr.com/photos/mv_virunga/sets/72157628609000883/
Hope that you find some of this of some interest.
If anyone reading this has any questions about the lifestyle etc. please feel free to chat with me on: firstname.lastname@example.org