1. Dear Pankaj, if you can show you have been in a subsisting relationship for more than two years with your girlfriend you can bring her into the UK on a partner visa. This visa, which will initially last for 2.5 years is given to partners who are in a subsisting relationship of two years where the British citizen has an income over GBP 18,600 PER ANNUM. As your rental income exceeds this figure, you would qualify. It makes no difference that you don't work.
2. Here is a link to the UK Government website which sets out the process of qualifying for such a visa and the application form https://www.gov.uk/join-family-in-uk/overview. The application can be made online.
3. If you don't have a two year subsisting relationship, you will have to apply for a tourist visa for her. This application would be made by her in Russia to the British embassy in Moscow. It would entitle her to spend six months on it in the UK. However, she could not work on this tourist visa.
4. Here is a link to the Government website where it sets out how to apply for a tourist visa https://www.gov.uk/standard-visitor-visa/apply. The application can be made online. Be aware that time spent together on a tourist visa can count as evidence towards showing a subsisting relationship of two years for a partner visa in the future.
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6. I regret to say your girlfriend won't be able to get married on a tourist visa. The registrar of marriages won't issue a marriage licence to someone on a tourist visa. You would have to get a visa specifically for the purpose of getting married. This would be for six months and shewould have to leave the country at the end of those six months and reapply in Russia for a further visa.
7. Secondly, you need to be paying tax on your income for it to be accepted for the purposes of a visa. You will need a tax receipt. An estate agent's receipt will not be sufficient.