If an individual sells part of their garden, the gain will be exempt from CGT so long as the total area of the plot including the footprint of the house is no larger than half a hectare in size. If the individual built a house on a piece of their garden and sold it, the profit would be subject to income tax and national insurance contributions on the basis that HMRC will say the owner is trading as a property developer. If, instead, they let the new property for a year or two, any gain would be subject to CGT instead. As part of the cost of the original property will need to be allocated to the new build, thereby diluting the main residence relief in the case of a development of part of the garden by the owner as opposed to a straight sale of part of the garden. If the main house was sold, the gain should be exempt from CGT subject to the rules for main residence relief being met notwithstanding the fact that part of the original cost will be allocated to the new build.
If you convert the stable into a furnished holiday let, part of the cost of the original property will need to be allocated to it and that will be added to the development costs as described in the previous paragraph. As you will be operating a business, any gain on the disposal of that business in the future will be eligible for entrepreneurs' relief so long as the rules for that relief are met and the relief still exists. Any qualifying gain would be subject to CGT at 10%. Any non-qualifying gain would be subject to CGT at 18% or 28% or a combination of the two rates depending on the level of the income of the individual making the gain. The gain on the disposal of the house will be exempt from CGT subject to the rules for main residence relief being met.
The only pitfall I can think of is to try and cut corners on valuations to save money. Any capital gain calculation on property in particular is subject to scrutiny by HMRC and they do use the services of the District Valuer who can challenge valuations, leading to costs for employing professionals to counter the DV arguments.
Take a look here and here for more information on main residence relief and CGT and the effect running a business has on it.
I hope this helps but let me know if you have any further questions.