A homeowners policy can be immediately effective for a new home during construction under certain conditions. This is often referred to as a "builder’s risk" or “course of construction” insurance. It provides coverage for homes that are being built, protecting against potential losses like theft or damage that could occur during the construction process.
In these situations, the insurance begins as soon as the policy is issued, even if the home is not yet fully completed. This coverage is vital as it mitigates risks inherent in construction, such as fire damage or vandalism, ensuring that the homeowner or builder is protected right from the start.
Other points in the question may relate to insurance commencing but often involve delays or specific requirements such as a finalized mortgage, completion of the home, or a necessary inspection. Therefore, the most immediate effectiveness of homeowners insurance relates directly to the construction phase when continuous risk exposure is present.