Like any Individual or Company Constituent, Mail Merge Letters can also be generated for Household records on the main Constituent Grid. Household letters can be generated individually or in a batch with other Constituent records.
One letter you may wish to generate is an End of Year Tax letter. Doing so with a Household would allow for all the transactions across each of the Household members to be included within a single document rather than multiple individual documents.
When generating mail merge letters for Households within a batch which includes other constituents, make sure you exclude any household members from the merge which will have individual letters generated as well. This can be done by checking the “Exclude Household Members” under the Filter section of the Cumulative Giving Options that appear on the Mail Merge dialog. This way you don’t have duplicate letters generated in your batch.