The house plans sold on The Plan Collection are drawn by leading architects and designers accross the country so they are quality plans drawn to code. As a home designer I can tell you that the codes are quite similar all over the country. In fact, recently the entire nation has adopted the IRC (International Residential Code) so all designers and architects are drawing their plans to meet or exceed these code rules.
The main thing you need to be aware of when purchasing a plan is that though the plans are drawn to meet the building code; different areas are subject to different weather and climate conditions. So a house plan that is drawn by a designer in the Northwest but being built in Florida will likely need to be reviewed by a local structural engineer. The reason being, a home in the northwest will need to be designed to support heavy snow loads. In Florida, snow loads are not a concern but homes do need to be engineered to withstand high tropical storm winds. So to make sure the home is built accordingly, the city will often require a structural engineer to review the plans. Even here in Northern Utah where I live; if I draw a plan for someone to build here, the city will require structural engineering before issuing the permit.
Short answer - the plans are drawn to code by professionals, but they may need to be reviewed by a local structural engineer depending on city requirements.