If you create an inventory of the items in your holiday home you can allocate a cost to replace them as new to get a total of how much you should insure for. This may take a little time to begin with but will be easy to update and manage going forward. Remember to keep the sum insured up to date when you buy new items or upgrade items in your holiday home. Keeping receipts and photos is also a good discipline and helps support any claims that you submit.
You could use a contents calculator as a helpful tool as there is one on directline.com used for our Home customers see the link below. Please be aware that this is for people's main residences and not holiday homes, so do not include items that are not covered such as valuables. Please be aware the bullet points about limits and cover relate to the Direct Line Home policy and not the holiday home insurance policy.