Depending on the level of work required, you can in some cases carry out all the work yourself, fitting kitchens, bathrooms, painting, new windows, moving interior walls, replacing doors, laying new floors, etc etc.
however if the work required means altering the main structure of the building, new second floor, large extention, etc, then you need to apply for planning permission,
if you are doing repairs to the building that does not alter the look or the construction, method or materials then this work too can be done yourself, if for example you want to replace an old roof with a new one, if you use the same materials (wood and tiles) then its straight forward to do. if yiu change to concrete then you need planning.
if the property has no water or electric connected then you need a project, planning to get them connected. watch out for planning time, it can take one year or more to get a planning project approved, regardless of what some agents, people will tell you. it does take a long time, also dont be tempted to carry out building work, alterations that should require planning permission, without getting the correct permissions, makes your house illegal and almost impossible to sell later.