In England and Wales, listed buildings are classified in three grades:
Grade I buildings are of exceptional interest, sometimes considered to be internationally important. Only around 2.5% of listed buildings are Grade I.
Grade II* buildings are particularly important buildings of more than special interest. Around 5.8% of listed buildings are Grade II*.
Grade II buildings are of special interest.
Some examples of Grade I listed buildings in the UK include Blackpool Tower, Portchester Castle and Liverpool Cathedral
If you live in a Listed Building (whichever Grade) and you wish to make changes to it you will usually need to apply for Listed Building Consent. Any work you wish to undertake will normally have to be agreed in advance. Often you have to apply for both Planning Permission and Listed Building Consent.
Internal modifications/alterations/ renovations to a Listed Building may not require Planning Permission but will still require Listed Building Consent. It is also worth noting that when carrying out work on a Listed Building there are Building Contractors that specialise in this type of work. Whilst they may seem more expensive initially when you get competitive quotations, in the long run they will often save you money by knowing how to source the correct materials as well as the correct methods that need to be applied when carrying out the work.
A designer that is familiar with both the Local Authority in your area, and who has worked on buildings of a similar Graded Listing, is always worth considering. They will be able to offer the best advice as to how to achieve a successful outcome with the Local Authorities, but also pay heed to your own requirements.