Since Happy Street's update 1.7, you need Helping Hands to upgrade your Friends Hotel, Museum, Modern House, and Seaside Hotel. To get them you have to connect the game to facebook in the «Friends» menu, and send request to your facebook friends who already play the game and are also connected to facebook.
Click on "Ask Friends" and the facebook pop-up will appear.
Here, search for your friends in the list but if your friends don't appear in this list, type their names on "to: Add some friends" to find them.
To finish click on "send" on the upper right of the facebook pop-up.
You can send requests to your friends every 4 hours to get new Helping Hands.
If you don't want to link your game to your facebook account or if you don't have facebook, you can also buy one Helping Hand for 20 Flooz.