Ownership of outbuilding

Hi I live in a mid terrace house which has an old outhouse /outbuilding across the street. The building is on the house title deed marked in blue and I also have a separate title for it. My new nextdoor neighbour now claims ownership , what is the definitive proof I will need to show as I fear it is going to end up becoming a dispute.

Hi Emma - if it's registered ten the register and title plan provide your proof of registered legal ownership 
If someone is trying to claim it as their own then they would most likely need to prove that they had taken possession of it 
Adverse possession of registered land (PG4) - GOV.UK

Hi Adam I'm not sure she is trying to do that , but she claims that her solicitor said that she owns it as it sits next to her parking space. Could it be a mistake on her title ? 

Hi Emma - you don't own something simply because it is 'nest to her parking space'. If the land is registered then the title plan and register would confirm ownership
Have you checked the registered information? - Search for land and property information - GOV.UK

Hi Adam I have had a look and her title doesn't seem to have been changed . I have also found 2 separate inspire id numbers which show the boundary quite clearly. I believe however that we will end up taking legal advice as she refuses to accept that our title deeds are correct.  Can I  apply for the paper documents online ? 

Emma - you can search for and view, download and save copies of the register/title plan online for a fee - Search for land and property information - GOV.UK
If you require official copies then you can apply by post - Get information about property and land: Overview - GOV.UK