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If you buy the new home with your partner before you sell the flat you own solely, you will have to pay the stamp duty surcharge I'm afraid. However, if you own a property which has been your main home "at some point" in the 36 months leading up to the purchase of your new main home, then you will be able to claim a refund of the surcharge so long as you sell that previous main home within 3 years of buying your new main home. See paragraph 11 of section 2.8 here (read 36 months for 18) and the answer to the question "What if I need to buy another main residence before I can sell my last one?" here.
In summary, you said that you left your flat in 2015 so whatever day in 2015 was your last day in that flat is likely to be within the 36 months before you buy the new home with your partner. You don't need to be moving "directly" from your previous main home but your previous main home must have been your previous main home within the 36 months leading up to the purchase of a new main home in order to qualify for a surcharge refund.
I hope this helps but let me know if you have any further questions.
I just answer the tax questions. I don't make the rules so I know the frustration and criticism isn't aimed at me. My tax system would be different to what we have now if I was in charge.
You can carry forward a rental loss for use in future tax years against rental profit but you cannot backdate rental losses to an earlier tax year I'm afraid. Once you sell the property, any unrelieved losses will be lost unless you own other rental property.
Would you mind rating my answer before you leave the site please.
Just a clarification. If you sell your property with unrelieved rental losses, you can use unrelieved losses if you start to re-let property within 3 years.