Hi Finnie!
I 've been staying at Add Mansion 3 years in a row! I'm always there in the low season (april), then I pay 15000 baht for a month, normally you'll have to pay about 1000 baht per night. It's a very nice and friendly place to stay, and not so far from either the beach (6-7 min walk), or Soi Bangla (15 min walk). They have some massage ladies downstairs if you want their service! They have two types of rooms all beautifully furnitured with big bathroom with hotwater, adjustable aircon , tv small fridge and phone. Front rooms are smaller with balconies, top floors more quiet. They have bigger rooms facing the backside but no balconies. I would recommend that you check out the rooms, before you check in, no need to book in advance!
I didn't stay there last time because because beginning to have some problem with noisy and broken aircon! Might have been fixed though!? I can also recommend you to check out several new hotels and guesthouses in the new soi( Soi Lamai Inn) behind Add or further down the main road! Some facilities and prices but newer. Have a nice stay
TJ