The cheapest type is a Velux or rooflight conversion, which is typically in the region of €18,000 to €30,000. It is the most affordable because it works within your existing roof shape rather than changing the structure.
In a Velux conversion, the roofline stays exactly as it is. Instead of building outward, roof windows are fitted flush into the existing slope to bring in light, and the space is insulated, floored, fitted with a staircase and finished as a room. Because there is no dormer box, no reshaping of the roof and usually less structural work, both labour and materials come in lower. Planning is also simpler in many cases, as Velux windows on the rear can often fall within exempted development, though you should always confirm this for your specific house.
The trade-off is headroom and floor area. A Velux conversion only gives you the space and standing height that your current roof allows, so it suits roofs that already have a reasonable pitch and ridge height. If your roof is shallow, you may get a pleasant room with sloped ceilings but limited full-height space.
If you need more headroom or floor space, a dormer or hip-to-gable conversion adds volume at a higher cost. For very tight roofs, our guide to small attic conversion costs is worth a read.
The honest answer for any roof is that suitability decides the cheapest viable option, not price alone. A free assessment will tell you what your roof can actually take.