Ok hi everyone - I've been tring to find out this for days and days and havent got any answer. That's weird because it's seems to be a pretty simple problem to me (which i cannot resolve unfortunately).

Lets keep it simple; I added a new house in an existing world (for example WhiterunWorld). I made a new interior Cell (for example NewInteriorCell), and added a door which links the WhiterunWorld to the NewInteriorCell.
For the player, there is no problem here, everything works fine.
However it is untrue for NPCs. I created a new NPC (lets call him NPC1) with a new AI Package, for example SleepPackage. In his SleepPackage, i told him to look for a bed in NewInteriorCell.
The problem is NPC1 doesnt seem to know where is NewInteriorCell because, ingame, when he is supposed to go to sleep into his house (NewInteriorCell), he leaves WhiterunWorld!!! And he walks for hours and hours on Tamriel's roads, until another package tells him to do something else, as if he had been searching NewInteriorCell the whole time.

So I understood every interior cell has a location, and a parent location. I created a location for my interior cell (for example NewInteriorCellLocation) and added a marker inside NewInteriorCell (and set its Location Reference type to LocationCenterMarker). I also put WhiterunWorld as its parent location, as every other Whiterun interior cells have. Then, when I try to edit NewInteriorCellLocation and put its LocationCenterMarker as the World Location Marker Reference, as described as in Location Creation Kit page (http://www.creationkit.com/Location), it doesnt work!!!! It seems impossible. Is this a bug or what???

My question is: how or what is the reference for a World Location Marker Reference??? I need to bind NewInteriorCell as a Whiterun Location, so NPC1 can understand that his house, NewInteriorCell, is actually in Whiterun!!!!
Is there another way to do that???

Some more questions: how do I add any Location Ref Type, Cells and Actors to a Location (edit window)?????

Thanks for your help guys! I'm quite desperate

Have you sorted out the navmesh for your interior and edited the one outside to accommodate your new building? NPC's wont use the door unless they are able to walk around on the other side

http://www.creationkit.com/Bethesda_Tutorial_Navmesh

- Hypno

Havent thought about that, I'll try it asap. Thanks for the tip!

*EDIT* ooh i feel so stupid, it worked!!!!!! And here i was messing around with locations and all that stuff....darn thanks a lot buddy!! I had hard time sleeping because of that, and it was only a navmseh thing... wow

Other questions: I am trying to make an NPC chop some wood - i made a package (copied from sigurd's in whtierun) for that purpose, gave the NPC an axe, but this same NPC just stands in front of the Wood Chop Block. He stands before it and looks at it, but he doesnt chop! what should i do??

Aahhh it worked for a bit, but now the NPCs started to do it again!!!
Even with a correct Navmesh, with the blue triangle in front of their door, they still sometime, when they should go into their house, they leave Whiterun!!!
Help...
*edit* nevermind, it seems when newly created buildings disappear, and you hit F5 to refresh, you must make sure the Navmesh is right again.

Also - my NPCs only enter their house when I leave the current world cell and come back. If i just watch them , they walk towards their huose and get stuck in front of their door. They do not enter except if I wait or like i said enter another cell and then come back at their home.
Why is that?

Navmesh is key here... make sure you have a connected node close the objects and close to the door so they can use it.