That's the main problem and what happened to me and I think also to many players unfortunately
These news is from wikia : http://elderscrolls.wikia.com/wiki/Lakeview_Manor
"Siddgeir may not be able to offer it either. If you happened to have already killed Siddgeir's housecarl, Heldvar, during the Dark Brotherhood quest, then this action will "cancel" out the chance to buy the deed from him or Nenya. This happened even when being Thane. You are unfortunately going to have to settle for the other 2 houses the Hearthfire DLC has to offer until there is a fix for this issue."