i'm no grammar expert but i think it's the difference between predicative and attributive expressions.
predicative: the house is old -> huset er gammelt
attributive: the old house -> det gamle huset
if it's an attributive statement in definitive form, you use the plural form of the adjective.
probably the easier thing to remember is just that if the adjective is in between 'den', 'det', 'dette', 'denne', 'disse' or 'de' and a noun, it should be in plural form.
This is for people who have this question who are seeing this thread, but I believe that liten, lita, lite, lille, and små are all used to describe small but are used in different cases.
Liten (m.), lita (f.), and lite (n.) are used for "general" objects that in english is basically "A..." ex. a small house = et lite hus
Lille is used for a specific object. In english which would be "The..." ex. the small house = det lille huset
Små is used for plural objects. Ex. the little houses = de små husene.
Hopes this helps anyone who sees this thread. :)