Improving Embassy Design and Security Act This bill amends procedures related to Department of State overseas construction. For any new embassy or consulate compound construction projects, the State Department may use a non-standard design only after consulting Congress. The State Department shall justify the choice to use such a design and provide documentation of the full lifecycle costs and the project's completion date, compared to the project if it used a standard design. The State Department shall report to Congress biannually on overseas capital construction projects; currently it is required to report annually on embassy construction costs. The bill also expands the required information for such reports, including the value of all requests to adjust the contract amount (such as a request for equitable adjustment or a certified claim). The bill also directs the State Department to complete all contractor performance evaluations by April 1, 2022. The State Department shall use the design-build project delivery method (where a single firm is responsible for both designing and construction) for all diplomatic posts and shall notify Congress if it seeks to use a different method for a project. The bill directs the State Department to report to Congress on various topics, including a six-year Long-Range Overseas Building Plan, a Long-Range Overseas Maintenance Plan, and a report detailing steps to expand the embassy construction contractor base to increase competition.