More and more Spaniards have been buying instead of renting their housing in recent years. Spain now has one of the lowest rental rates in the EU. Low salaries and expensive housing mean that most Spaniards are living at home longer (late 20s/early 30s) and then buying a home rather than joining the rental market.
The dramatic rise in property prices over the past several years has lead to high competition for available rental properties. Monthly rent for a 1 bedroom furnished flat starts at €600/month in Madrid and Barcelona. A cheaper option is to look for a room in a shared apartment (piso compartido), which may start around €300/month in the major cities.
May to July is the best time for flat hunting; August can be difficult as people are on holiday. September tends to be the worst month as people return from vacation and students start a new year. The housing market can also be slow over the Christmas period.
Finding a good apartment at the right price before you arrive in Spain is difficult. Landlords prefer to meet tenants before signing a contract and the wide range in quality means it is not advisable to commit to rent a flat "unseen". If you require your living arrangements to be organized before you arrive, you can use an apartment rental service. However, these do tend to be expensive.