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To get your first house, you need 1000gp. Go to the Estate Agents in Varrock, Seers Village, Falador, or Ardougne and buy a house. Go to the Portal in Rimmington and you can enter it. You can also talk to the Estate Agent to remodel your house's walls or move it to a different house portal. It's convenient to have a house in Rimmington because of the Pick Axe spawn and the clay mine.

[edit] Buying other supplies

The Sawmill Operator is a useful NPC to know when you need to buy various house supplies. He is at the Lumber Mill where you search for the cat in the quest Gertrude's Cat. He sells nails, saws, and cloth and will turn your logs into planks for a small fee depending on the type of log.

The Normal Planks that you can have made from Logs here are used in some quests. They are used in quests such as Lighthouse, Spawns in the Wilderness and north of the Barbarian outpost.