Thanks for your enquiry.
You are correct in that the Deeds for the 7 properties need to be in your joint names to enable you both to utilise your CGT annual allowances.
The problem you have is that the properties all have Mortgages on them. Hence, before any of them can get transferred into joint names, your Wife will need to approach each Mortgage Lender for their consent to transfer the Deeds into joint names, and they will only consent if the Mortgage is also transferred into your joint names (usually by way of them issuing a new Mortgage Offer).
The legal cost for dealing with each Transfer is likely to cost approx. £600 plus VAT, and the Lender will also normally make an admin fee of approx. £200.
On top of this, there is also a Stamp Duty issue. Each Transfer will be subject to Stamp Duty at the enhanced rate, in that if your Wife transfers the property into your joint names, you are deemed to be paying consideration equating to the value of one half of what is outstanding under the Mortgage (you are assuming the debt of half the Mortgage). So, for example, if 1 of the properties has an outstanding Mortgage of £100,000, you will be deemed to be assuming a debt of £50,000, meaning that Stamp Duty of £1,500 will be payable (the rate is 3% up to £125,000 and then 5% up to £250,000).
With this in mind, it may not be a financially worthwhile exercise!
I hope this assists and sets out the legal position.
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