The sentence "If the house was less expensive, we would be able to buy it" should be corrected by changing the verb to be "was" for "were," in the frist part of the conditional sentence. So that it would be: If the house WERE less expensive, we would be able to buy it.
The second conditional uses the past simple after if, then 'would' and the infinitive:
if + past simple, ...would + infinitive
We can use 'were' instead of 'was' with 'I' and 'he/she/it'. This is mostly done in formal writing.
