Leasehold improvements include anything that needs to be done to the space that you are going to lease in order to make it fit for your purposes. Your lease should outline who is responsible for making the changes, who is paying for the changes, and what the timeline is for getting it done. It should also make clear who is the decision maker with respect to the leasehold improvements. For example, if new flooring is to be laid do you get to decide the specific type or does the landlord.