The biggest unknown that is hard to budget for is in the area of permits. You are at the mercy of the town and when they can process the application and get an inspector out if needed. If you happen to deal with a historic property (we made that mistake once buying a property in a historic district), that is an additional wildcard. An inspection by a "hard ***" can shut a job done instantly and that cascades to your trades that have your job slotted in their schedule. Dates move and your trades move on so the delay cascades. It will impact your borrowing costs as well especially with a HML. Doing some homework upfront to see what the timeline tends to be in the area you are looking at with the town / city is time well spent before you find that perfect property that winds up eating up all of your profit in delays and increaed carrying costs.