Reward: 7 in-game days crafting boost for your workers.
Note by doenen: As of the (October?) update that introduced new racks, you do not require pet upgrades to get the Decorator achievement. You only need to have fulfilled the appropriate number of upgrades required for the achievement (not sure what the exact number is but I got Decorator after finishing several upgrades from the December update despite only having 1 pet, and a number of my guild members who have no pets have also gotten the achievement after completing the new upgrades)
Go to Achievements list