Home extension vs block garage for home office

Hi,

We plan to build a detached block garage or add a rear extension to our house to use as office space as we have been permanently switched to remote working.
I was wondering which option would be best with regard lowering our LTV ratio, as we have 20+yrs left on our mortgage.

The latter is likely to increase value more and be more energy efficient

In the depths of winter, not having to venture outside to an office will be a winner.

An extension to the house will likely cost a little more to build, but done right it will also add more value to the house. If can be tied into the main heating system, saving on heating costs, and can adapt as your needs change.

Future buyers not looking for an office might like a playroom or extra bedroom or living room, and those who do want an office might value being close to the kitchen or a bathroom or being in the house when there are kids about. An office built in a garage will only really have value to people who want a separate office.