The Property Investors Federation says neglecting maintenance work could prove costly for those who are putting it off.
A Building Research Association survey has found that the number of homes in good condition - 41% - is down from 50% in 2005. Rentals were found to be in a worse condition than owner-occupied homes.
While acknowledging that the decline will be due to tough economic conditions, the federation says people need to look after their investment.
Immediate past president Martin Evans says even a coat of paint can save money in the long run - and the longer you leave it, the harder it is to do.
The better a property looks, he says, the easier it is to get a tenant.
Mr Evans says the federation is advising its members to spend what they can to give their properties an edge.