If you're building a house, you don't need a housecarl, you need a steward. Those are two different things. You get a housecarl when you become Thane of an area and/or buy a home or plot of land (usually you have to buy a house to become Thane or become Thane to buy a house). You can ask your housecarl to be a steward, but I don't recommend it. Only because it's nice to have two people, rather than just one, patrolling and protecting your property. The Hearthfire homes get attacked A LOT, so the more defenders you have, the better. (As they can also be followers, make sure to get them to follow you at least once and put lots of good stuff into their inventories, armor, weapons, healing potions, enchanted ring, staffs, etc, so they'll be more powerful when you're not around).
Anyway, it's the steward that can buy materials and stuff for you, like wood, chickens, horses, etc. You get a steward by taking certain followers to your property and getting the dialog option to ask them to stay as your steward. Only certain NPCs can become stewards, not all of them. For a list, see: http://www.uesp.net/wiki/Skyrim:Personal_Steward