Post office returned the notice to builder

Since you have sent the notice to the builder is enough. It seems he had manged the post man.  Though it was returned you can file consumer case by enclosing the returned cover.  But the address should be correct.  The Consumer court will again send the summons to the Builder, if the builder still manages to return the summons, substituted service will be served and then set exparte. 

You are entitled to get refund of amount paid with interest for failure to give possession within specified time or further time allowed. The developer fails to give possession in accordance with the terms of his agreement of a flat duly completed by the date specified,or any further date or dates agreed to by the parties.The amount and the interest shall be a charge on the land and the construction,if any,thereon in which the flat is or was to be constructed to the extent of the amount due,but subject to any prior encumbrances.

Thanking you, 

The returned  undelivered registered letter may be filed as a document for having served a notice but not accepted by him in the consumer case proposed to file against the builder.  Yes, you can proceed with it.