There is a difference between attaching things to a house and attaching things to an implement. One may not add anything to (or remove anything from) a house even if it is attached loosely, since the house is considered permanent. For example, one may not install a window on its hinges or remove it from its hinges. Although it is attached loosely and it is easy to make or break this attachment, it is forbidden because the window is part of the house. Note that if a wardrobe or free-standing closet has a volume of forty se’ah (three cubic amot, or just over ten cubic feet) it has the same status as a house. Thus, one may not connect anything to a large wardrobe, even loosely (Rema 314:1).
In contrast, devices, which are less permanent, may be assembled using temporary attachments that do not require craftsmanship or strength. Therefore, one may assemble and disassemble a portable cot of the type travelers use, which can be assembled and disassembled daily, and which is loosely attached (Shabbat 47a-b; SA 313:6). Similarly, one may remove the door of an implement that is easily assembled and disassembled and that turns easily on its hinges. Technically, one should be permitted to place this door on its hinges as well. However if the hinge is attached to the implement with nails or screws, the Sages forbid it out of concern lest one tighten the attachment and thereby transgress a Torah prohibition. Only if nothing can be tightened, such as if the hinge is part of the implement, may the door then be attached to the implement (Shabbat 122b; SA 308:9).
The general principle here is that if an implement is normally assembled with a firm attachment, the Sages forbid even attaching it loosely. They were concerned that one might forget and attach it firmly, thus transgressing a Torah prohibition. However, if an implement is assembled with temporary and easy attachments, there is no reason to fear that one will attach it firmly and thus transgress a Torah prohibition, and it may be assembled and disassembled. Therefore, one may extend a table by adding a leaf designed for this purpose, and one may remove a leaf as well, since the leaves are clearly temporary. Similarly, the feeding tray on a child’s high chair, which is constantly attached and detached, may be used freely on Shabbat, since the attachment is loose.
Based on this, many maintain that the prohibitions of Boneh and Soter do not apply to building with interlocking toy bricks (like Lego) and the like, since the pieces are attached to each other temporarily and are designed to be disassembled (Tzitz Eliezer 13:31; Yeĥaveh Da’at 2:55; but SSK 16:19 is stringent).
Some Aĥaronim maintain that it is rabbinically forbidden to make paper airplanes or boats or to fold napkins into special shapes because this resembles Boneh (R. Shlomo Zalman Auerbach cited in SSK 11:41; 16:21). Others permit this on the basis that the prohibition of Boneh does not apply to things that are temporary and will soon be disassembled (Rivevot Ephraim 1:223:8 quoting R. Moshe Feinstein). One who is lenient has an opinion to rely on, and one who is stringent is commendable. Children may be lenient even le-khatĥila (see Harĥavot).