It may not be DIFFICULT, it may instead be LONG AND TEDIOUS. They are dedicating their time and money in new content instead of adding a lot of customized code in order to add all of the new space into the houses. Time and Difficulty are not necessarily exclusive when talking about programming. I understand this and I've only dabbled in coding and such in things like writing out Python, Ruby and HTML and simple codes, now going into a full game code there are even more additional lines of code that need to be cross referenced and typed out to make sure that you don't have the same item slot read or contain two different items or cross with a different house...

I mean they probably have to add the code on EVERY HOUSE in EVERY ZONE and edit it to be uniquely categorized. So it's not hard to do it as it's something they've already done, they just add more lines of code, BUT ensuring it's all uniquely recognized is what makes it a time sink.