Top-Down Design

In top-down assembly design, one or more features of a part are defined by something in an assembly, such as a layout sketch or the geometry of another part.

The design intent comes from the top, which is the assembly, and moves down to the parts. Examples of design intent include feature sizes, component placement in an assembly and proximity to other parts.

For example, you create a locating pin on a plastic part using the Extrude command. You choose the Up to Surface option and select the bottom of a circuit board, which is a different part. This selection makes the locating pin long enough to touch the board, even if the board moves in a future design change. The length of the pin is defined in the assembly, not by a static dimension in the part.


You can use these top-down methods:

  • Individual features can be designed top-down by referencing other parts in the assembly like the example of the locating pin.

    In bottom-up design, a part is built in a separate window where only that part is visible. However, you can edit parts while working in the assembly window. This makes the other components' geometry available to reference. Examples of geometry to reference include copy or dimension to.

    This method is helpful for those parts that are mostly static but have features that interface with other assembly components.

  • Complete parts can be built with top-down methods by creating components in the context of the assembly. The component that you build is attached or mated to another existing component in the assembly. The geometry for the component that you build is based on the existing component.

    This method is useful for parts like brackets and fixtures, which are mostly or completely dependent on other parts to define their shape and size.

  • An entire assembly can be designed from the top-down by first building a layout sketch that defines component locations, key dimensions, etc. Then build 3D parts by using one of the methods above, so the 3D parts follow the sketch for their size and location.

    The speed and flexibility of the sketch allows you to try several versions of the design before building 3D geometry. Even after you build the 3D geometry, the sketch allows you to make a large number of changes in one central location.


  • Whenever you create a part or feature using top-down techniques, external references are created to the geometry you referenced.
  • In some cases, assemblies with large numbers of in-context features, which form the basis of top-down design, can take longer to rebuild than the same assembly without them.
    SOLIDWORKS is optimized to rebuild only changed parts.
  • When creating in-context features, do not create mating conflicts because these conflicts can cause long rebuild times and unexpected geometry behavior. You can avoid these conflicts by not creating mates to geometry that is created by in-context features.