The best type for you will depend on your specific roof structure (or preferred mounting location), budget and space required. The different solutions explained:
Velux (Roof Light) Conversion
Velux: The easiest and cheapest way of converting the room with this style is by adding windows to your existing roof slope without making any structure changes. Great for when you already have plenty of head height. They cost much less and can normally be constructed under permitted development, but offer the least extra space.
Dormer Conversion
Dormering is the primary option; with dormers extending down vertically from your roof slope, they can add that little bit of extra headroom and floor space. Rear dormers usually do not require planning permission and work well on most property types. They are great for the money and produce a ton of usable space.
Hip-to-Gable Conversion
Semi-detached or detached house types with hipped roofs (sloping in on all sides) can push that plane up into the roof creating a traditional vertical gable wall and getting everything possible out of every square inch of volume. Many times accompanied by a rear dormer to utilise full room size. Additionally, the extra space it will create is significant. For Loft Conversions Exeter, contact Silverfern, providers of Loft Conversions Exeter.
Mansard Conversion
A mansard conversion actually alters the structure of your roof to create an almost vertical wall and flat top, but how far this protrudes depends on you! They offer the most floor space and greatest headroom but usually will require planning permission. Ideal for urban lots where getting the most out of every square foot justifies spending.
Making Your Choice
Think of your budget, type and strength of the existing roof structure and planning constraints. Once you have answered these questions, then consider consulting a structural engineer or loft conversion specialist to ascertain what exactly is possible for your property and narrow down options before making the final decision.