Hello and thanks for using Just Answer.
My name is Al and am happy to assist you with your enquiry.
As regards the Land Registry title plan for the property, this is likely to be a Plan from the 1970's or thereabouts, and therefore it does not matter one bit that the annex is not shown. What is important, is that the whole extent of the land that you purchased and which you possess is shown on the title plan.
As rgeards the annex, your Solicitor should have asked the Sellers Solicitor at the time of your Purchase for a copy of the building Regulations Completion Certificate.
If this Certificate had not been issued, he should have fully advised you on the implications as well as notifying your Lender.
If the work had been carried out some years before you purchased, the Seller may have offered a "Building Regulations indemnity policy" which basically acts as an Insurance policy for you, should the Council ever query the annex and ask that certain work is carried out by you to bring it up to Building Regulations standards. If such a Policy was taken out by your Seller and your Solicitor also advised you on the Policy as well as confirming that the annex did not hold a Completion Certificate, then he has covered his own back, and you have no claim against him.
If no policy was issued and your Solicitor at the time did not advise you on th eimplications of proceeding without the COmpletion Certificate, then he has been negligent and you would have a claim against him for any costs you now incur in putting right what the Council want you to do in respect of the annex.
You should therefore see what your Solicitor comes back with, and take it from there.
I hope this assists you, but please let me know if I can be of any further assistance.