Of course since we have legally declared the asset on the 720 form , we have had to declare that it is sold. From the information coming through it doesn't look too good.
But the figures actually are purchase price £141,000 and selling price £165,000. However, we spent £22,000 on an extension but like the example shown, we have no receipts or at least I can't find any although we did have them at the time from the builder. They were in a folder at the house but seem to have disappeared from the loft where they had been stored. There are details of planning permission etc though and if the builder is still in business maybe we could obtain copies.
On that basis it would seem there would be very little tax to pay anyway.
My understanding from my Spanish accountant is once you have submitted a 720 form you only have to submit another if your assets in any of the categories change significantly so yes you would have to submit another form making hacienda aware you have sold the property.
As you say the key here is receipts for the improvements that should solve the problem.
The reason I originally posted on your thread was that I am living in Spain while renting my UK house originally with the intention of returning but things have changed and I want to live in Spain. The profit on my house is quite large and recent changes to CGT laws in Spain mean I can't even reduce the taxable profit by the time owned I get hit on the entire gain. I was hoping there was another way but since taking further advice there appears not so I will deregister in Spain and live in the UK for a while.
Good luck with your situation looks like you have a solution.
Regards