Thank you for your question.
Gain made from sale of lock-up garages would be chargeable to capital gains tax.
You would claim gains annual allowance of £11,300 against the gain of £20,000 less cost associated with buying and selling and the balance would be subject to CGT at 10% provided your total income including the gain after allowance remains within £33,500 threshold for basic rate of tax.
Say your other income after personal allowance £20,000
Gain after gains allowance is (20,000-11,300) £8,700
Total income (20,000+8,700) £28,700.
This is below £33,500. Therefore your capital gain would be taxed at 10%
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.