The addition

The goddess, flashing-eyed Athena, put it into the mind of the daughter of Ikarios, the judicious Penelope, to set up the bow and the gray iron in the feast-hall of Odysseus, to be a contest and the beginning of death. She went up the steep stairs of her house, and she took the bent key in her strong hand-beautiful, made of bronze, and the handle was of ivory. Then she descended from her chamber with her attendent women to a storeroom at the far end of the house, The treasures of her king lay there, bronze and gold and well-worked iron.,

Odyssey Book 17 line 246-248 Barry B Powell
Odyssey Book 21 line 1-9 Barry B Powell

The 'addition'

There's actually a hint about the addition, when Homer writes about Penelope's clever idea to set up a contest.

Homer mentions here a storeroom at the far end of the palace, which could very easily be the two-storey addition at the northern wall. This would mean the storeroom isn't an addition but just a part of the ancient palace, in which case the length of the palace would come to a total of 70 metres.