If you look at CG64950 in the HMRC manual here, you will see that unless the property is your only or main home at the time of a transfer into joint names, your husband's period of ownership will begin on the actual date of such a transfer as opposed to your original date of purchase. He will not, therefore, qualify for main residence relief or letting relief on his share of any gain if you do put the property into joint names before you sell it, assuming that you don't move into it that is.
If you sell the property in, say May 2014, for £480,000, you will make a gain of £370,000 (£480,000 - £70,000 - £40,000). By that time you will have owned it for 316 months of which you will have lived in it for 72 and let it for 244.
Based on your ownership period to, say May 2014, of 316 months you will qualify for exemption from Capital Gains Tax for £105,380 (£370,000 / 316 x 90) of the gain as you will be given the last 18 months of ownership as an exempt period in addition to your 72 months of occupation.
The remaining gain of £264,620 will be that part of the letting period gain not covered by the last 18 months of ownership. As the property will have been both your main home and let you will be entitled to letting relief which will be the lesser of:
2 the sum of the main residence gain and the gain for the last 18 months of ownership of the property which is £105,380 and
3 the letting period gain of £264,620.
Letting relief of £40,000 will reduce the remaining gain of £264,620 to £224,620 and the annual CGT exemption of £11,000 will reduce it further to leave you with a net taxable gain of £213,620.
There are two rates of CGT, 18% and 28%. The rate or combination of rates you will pay will be dependent on the level of your income in the tax year of disposal of the property. Assuming you sell the property in the 2014/15 tax year, one of the following scenarios will apply:
1 If your income in 2014/15 including the taxable gain is £41,865 or less, then all the taxable gain will be taxed at 18%.
2 If your income in 2014/15 excluding the taxable gain is more than £41,865, then all the taxable gain will be taxed at 28%.
3 If your income in 2014/15 excluding the taxable gain is less than £41,865 but more than £41,865 when you include the taxable gain, then part of it will be taxed at 18% and part at 28%.
If you put the property into joint names, you will each have a gain of £185,000. You will qualify for main residence relief of £52,690, letting relief of £40,000 and the annual CGT exemption of £11,000 leaving you with a net taxable gain of £81,310. Your husband will have a net taxable gain of £174,000 (£185,000 - £11,000). The additional taxable gain will be £41,690 (£81,310 + £174,000 - £213,620). You would need to consider your respective tax rates to assess the effect of a transfer into joint names.
You may choose not to claim main residence relief for the nine month period that you lived in the property during the week in 2003 as this would impact on your claim for main residence relief on your other home when you sell it.
Read the article here for a very aggressive way of attempting to avoid a taxable gain. HMRC will almost certainly challenge such a move as aggressive tax avoidance and seek to disallow the claim for main residence relief.
I hope this helps but let me know if you have any further questions.