First you will want to do a ‘walk around’ in your yard and take a look at the tree(‘s) or stand of trees available. The design of the treehouse structure will to a large extent depend on the size, shape and species of tree you have to work with. If you are fortunate and have several trees, or a grove from which to select from then strength, vigor, species- (good compartmentalisers), form, and arrangement are of primary concern.
Right away we’ll want to educate ourselves about some of the aspects of a tree well suited for constructing treeforts, treehouses or other treestructures.
Species is important, and knowing your local varieties strengths and limitations can be researched to ensure that you pick the best candidate in your treeserve. Individual trees are subject to analysis to ensure fitness for building treestructures in them. Ask yourself, which are the oldest and largest species in my area. Which ones have the tops blown out of them, or limbs twisted off. Also, think about lightning if you live in an area where lightning strikes with some regularity. Consulting an arborist is one way to go for analysing a tree on your property. Local oldtimers in your area can be a good source of info. regarding specifics of which species varietals are the most hardy, strongest, long lived and resistant to storm damage. Or you might want to talk to the Entreepreneur himself, Michael Garnier.
Compartmentalization is another trait that a good treestructure tree should have in it’s bag of tricks.

Standards: Modern day Treestructures.
In the “old days,” one didn’t have to be as particular about how, or where to build a treestructure. Basically any tree or location one chose was ok. Not now, In this “New Age,” safety standards and neighbors’ privacy have come into play. Building Depts. And Planning commissions have been become more concerned about how and where treestructures are built and the particulars of where they are located. Also how the treestructure is going to be used. Although building departments in some areas have not addressed the “treestructure” issue with any adequate rules, many are now comming up to speed. If you’d like some advice in regards to dealing with these sometimes difficult authorities, talk to some of your local building contractors. They deal with the county officials on a regular basis and know many of the ins and outs of dealing with them, as well as basic construction codes in your area.
If someone living in your neighborhood decides to complain about your hastily started project, or your local Building Department personel or Assesor happen along you may find yourself suddenly in the middle of a legal battle, as was the case of Michael Garnier of Treesort, “Out ‘n’About,” in Takilma, Oregon.
In closing, be sure to talk to as many of your neighbors as possible before you start a treestructure project on your property. People living in your immediate visual contact zone may feel a sense of intrusion if they can see your treestructure from their windows or yard. Another thing about the intrusion issue is whether any of your treehouse windows face a neighbors backyard or windows. This can and does occasionally bring complaints to authorities and they may end up on your doorstep. Better to have some idea about how individuals in close proximity feel before you start your project
Ok, now we’re cookin’. Lets get on down the list of ‘to do’s’ of selecting the proper tree, or trees on your property. There are several parts of a tree to consider. Since you cannot dissect a tree you are going to use, it must be scrutinized from the outside.

Roots: The foundation.
Look for diseased tissue where the trunk meets the ground, such as fungus / conk, or damaged bark. Compacted earth over tree root area. Too close to driveways or heavily used foot paths or filled ground inside of drip line can produce damage to roots. Have an arborist or other qualified treeperson analyse the roots if your tree is growing where these conditions exist.

Trunk: Foundation above ground.
Inspect now, for rotten heartwood due to die back or wind damage of old limbs. Sometimes trees are damaged by lightning, freezing or extended drought. Look for large openings in the bark where the tree is trying to heal over some past damage and see if the exposed wood is rotting or still solid, or if compartmentalisation is working to allow healing. Trees rotten in the core are oftentimes visibly hollow. If no hollow is apparent then one can take a length of 2×4 lumber and tap on the trunk. Listen for changes in tone that may signify hollows or changes in density of the trunk wood.

Branching: Another aspect of selecting a tree.
The natural branching habits of trees vary. The strongest limb attachment is the 90 degree angle. A 45 degree angle is ok, but limbs with less angle tend to have included / invaginated bark which weakens the joint between forked limbs, or limb and trunk. Also included bark may allow high winds to tear the tree apart at the weak spot. There are ways of dealing with trees that are not at first site the best looking.
The GL’s© and other associated Bracketree© will enable a person to take advantage of precisely placing your treestructure in the best possible orientation in the tree, as well as being able to take advantage of what might otherwise be seen as a not so useful tree.

General info. only, not an endorsement for these species.

Trees with 90 degree limb angles:

Oaks; (White Oak – Quercus alba) (Post Oak – Q. stellata) (Bur Oak – Q. macrocarpa)
American plane tree / Sycamore – Planus occidentalis.
Tamarack / eastern Larch – Larix laracina.

**note! almost any tree can sprout new limbs on a seconds notice using new techs. GL’s.

Trees with 45 degree limb angles:

Oaks; (Black Oak – Quercus velutina) (Scarlet Oak – Q. coccinea)
Maples; (Red Maple – Acer rubrum( (Silver Maple – A. saccharinum) (Sugar Maple – A. saccharum)
Hickorys; (Bitternut Hicory – Carya cordiformis) (Pignut Hicory – C. glabra) (Shagbark Hickory – C. ovata) (Pecan – C. illenoensis)
Walnuts; (Eastern Black Walnut – Juglans nigra) (Butternut (walnut – J. cineria))