Take a look at HS283 here, example 9 in particular.
Whether you have to pay CGT or not will depend on the facts and figures of your case. If the property was ever your main home and you lived in it, you will be entitled to exemption from CGT for the proportion of the gain covered by your occupation of it. In addition, you will be entitled to relief from CGT for the proportion of the gain covered by the last 18 months of ownership. You will also be entitled to letting relief if the property was ever your main home and you lived in it. If it was never your main home, then all the gain will be taxable subject to exemption for the first £11,100 (current tax year).
Assuming you don't already complete tax returns, you will need to do so for the tax year in which you sell the property and submit it to HMRC by 31 January following the end of that tax year if the disposal proceeds for all asset disposals exceed £44,400 and/or the gains exceed £11,100 (current tax year figures). CGT is payable on 31 January following the end of the tax year in which the gain is made. The rates of CGT on property gains are 18% and 28%. The rate or combination of those two rates that you will pay will be dependent on the level of your income in the tax year you dispose of the property. There is information on CGT here.
I hope this helps but let me know if you have any further questions.
Can you elaborate on what you mean by "so would like to know at which point these assessments are made? My solicitor, the buyer's solicitor, the estate agent etc.". Assuming you have owned the property for 3 years and six months, can you me what it cost to buy and what it is worth now.
You declare the gain in your tax return. The solicitors and estate agent don't get involved in your tax situation.
It's impossible to tell how long you would need to live in the property as that is dependent on how property prices move and the tax rules in place at the time of disposal. You get the last 18 months of ownership as a tax free period in any event so you would need to lived in it beyond a further 18 months to make any difference at all. That's why I asked for the purchase price and current value.
Let me do some calculations and I'll get back to you.
Total period of ownership: 42 months
Owner occupied: 6 months
Let: 36 months
Gain: £300,000 (£550,000 - £250,000)
Exempt gain: £171,429 (£300,000 / 42 x 24 (6 + 18))
Non-exempt gain: £128,571 (£300,000 / 42 x 18 (36 - 18)
Letting relief: £40,000 (lesser of £40,000, £171,429 and £128,571)
Annual CGT exemption: £11,100
Net taxable gain: £77,471
You can deduct the costs of buying and selling (legal fees, survey fees, selling agent fees etc) from the gain.
The costs of letting and a new boiler should have been deducted from the rental income.
They are revenue as opposed to capital expenses. I assume you have been declaring the rental income.
Take a look here for information on the types of expenses you can claim against rental income.