Ancien = former / old (adjectives that change meaning according to position)

are these two the same meaning?

No, they mean different things. "L'ancienne maison" is "the former house", whereas "la maison derrière" is "the house behind" (spatially speaking). 

Hi James ,

 'Ancienne'  in your example means former/previous so you might say .

"Dans mon ancienne maison j'avais cinq chambres "= In my previous/former house I had five bedrooms.

In my opinion, if you say: "Dans ma dernière maison", it sounds as if you are a serial mover and it is the last one in a long list ...

Hope this helps!

Sorry, I mesread derrière for derrière. 

Zut! DERNIÈRE for DERRIÈRE. 

Well, it could be a lot of things, I guess. The point being to demonstrate the change in meaning ancien has when going before or after the noun. 

-- Chris. 

So my answer might have been correct after all, but I appreciate the difference. Thank you for your prompt responses which are appreciated 

"Thought of a simple way to keep straight how "ancien/ne" changes meaning:
When beFore a word, it means Former.
Not the most graceful mnemonic in the world, but it helped until I had mastered the difference."

And when 'A'fter a word, it means Aged. :)