Use of "en"

Question

I don't really understand why "en" is used in this example, even after having reviewed the lesson you mention. Where does "de" come into it? "If I had a house", for example, would be "Si j'avais une maison" - not "Si j'en avais une maison, surely? In this case, the use of "en" seems to be rather random!

Accepted Answer

Bonjour Martin !Here it's because the full expression would be "avoir les moyens de [faire quelque chose]" = to have the means to [do something]. I hope that's helpful!À bientôt !

Answer

Bonjour Martin !

Could you please send me the sentence you're referring to? 
I'd be happy to assist :)

À bientôt !

Answer

Merci Aurélie.  The sentence is : "J'aurais une grande maison si j'en avais les moyens." I understand the use of "en" where "de..." occurs before, but that doesn't appear to be the case here.
Cordialement   - Martin