For a loft conversion, it is generally accepted that you do not need a building permit as long as it meets certain requirements. The volume of the house can not change, meaning that the building will not grow larger from the conversion.
You can not build anything above the current roof, such as a dormer, a structure containing a window. Make sure that your house is not listed or on a conservation list.
If it is, your property will need to be looked over by the local preservation society before work can begin. To be safe, always check with your local government to make sure you do not need a permit.
Before deciding to build, you must make sure that the roof of your house allows a decent amount of conversion space. Measure your roof from the tallest vacant point in your loft to the floor.
If it measures less than 7 1/2 feet, you will not have sufficient space to build. Take a look at the rafters in your roof as well.
If it is a rafter roof, containing no support, you will need to add extra support to hold the weight of the conversion. A truss roof is easily converted.
You must find room for a staircase to your loft conversion. According to most building regulations, a fully functional and sturdy staircase is required to access the loft conversion.
Pre-existing ladders to the loft do not count as a suitable substitute for an actual staircase. If you are using the loft for commercial purposes, you will have to put in a staircase.
If the loft is for your own personal use, it is still advised, as building regulators can slap you with fines if a conversion is not considered safe. Check with your local government for your areas specific regulations.