TREEHOUSE BUILDING TIPS

* Choose your tree(s) carefully. Maple, oak, fir, and hemlock are good. ''Black walnut is notoriously brittle,'' builder Peter Nelson says.

* A treehouse's shape is dictated by the tree as much as by the builder.

* Allow ample time. If building something on the ground might take twice as long as you anticipate, figure three or four times your expectation for a treehouse.

* One key to safety is strong railings.

* Build as much as you can on the ground, then assemble it in the tree.

* ''Trees bend, and treehouses must bend with them,'' Mr. Nelson notes.