The cost of a loft conversion or extension can vary widely based on several factors, including the size of the project, the materials used, the level of customization, and your geographic location. On average, a basic loft conversion may cost between £20,000 and £40,000, while more complex conversions with high-end finishes can exceed £50,000. Extensions typically range from £30,000 to £70,000, depending on their size and specifications. It's advisable to get a detailed quote from a reputable contractor to understand the costs specific to your project.