Some buildings are designed to be built in the wilderness, and carry costs that reflect that. While they are shown on their respective pages, they are reproduced here for good measure.
If you want to claim a bit of wilderness as your own estate or holding - start here. Probably a Bandit Camp first, which allows you to develop a redoubt. That would give you protection as you built your fort … I would probably salvage what I could from the bandit camp, and incorporate it into the fort. It won't reduce the cost of the fort by much, but it will save a bit and means you need fewer soldiers …
However, note that these are military buildings, and you would have to recruit troops to staff them - or hire mercenaries.
Treehouse (3,440 gp) - Treehouse strongholds are particularly popular among small communities living in wildernesses so dangerous that sleeping on the ground is never safe.
Bandit Camp, Large (4,270 gp) - This semipermanent camp includes a few simple wooden structures hidden from the view of prying eyes by Blinds, allowing dozens of bandits to operate in the wilderness year-round with relative security.
Redoubt (5,150 gp) - The reinforced, highly defensible structures built into the natural terrain known as Redoubts are often constructed to take advantage of locations such as cave entrances, mountain passes, or river fords.
Druid’s Grove (5,750 gp) - This is a refuge in the wild where spellcasters tied to nature can enjoy shelter and practice their crafts in outdoor surroundings.
Fort (6,050 gp) - A fortified outpost for bandits, mercenaries, soldiers, or dangerous humanoids.
Witch Hut (7,500 gp) - Be it a crazy old hermit, a hag, or just an herbalist who likes to live near the source of her craft, a spellcaster who prefers to dwell far from civilization calls this structure home. (*)
A few other buildings might be appropriate - but they will cost at least twice as much as building them anywhere else.