Loft conversions can be a complex matter.
This means that you must be very sure of your DIY skills and knowledge of proper building methods before attempting one yourself.
It may also be a good idea to consult with an architect before contacting a
loft conversion company.
Make sure you obtain written quotations from at least two loft converters.
Estimates are not good enough. Ask each company for references from other projects similar to yours.
You should also check the financial solidity of potential suppliers and make sure that you agree payment in a way that does not leave you vulnerable to delays or shoddy work.