Let's say, the nouns have all symbols. In this case, the symbol for 'Haus' is 's', from the article 'daS'. The point is that a symbol is used exactly one time, never more often. That is why you have 'das große Haus', but ein großes Haus. Compare for masculine, where the symbol is 'r': der große Mann', but 'ein großer Mann'. For feminine, it looks slightly different because of the 'e': 'die große Frau' and 'eine große Frau', but actually it is not.

It's "ein großes Haus", but "das große Haus". German adjectives have several conjugation patterns depending on what precedes them, such as definite or indefinite articles – see, for example, http://www.learn-german-smarter.com/learn-German-adjective-endings.html