Converting detached garage

[runner2]

Our single garage is separate to our house and we’re considering converting it into an annexe; our priorities are a work/office space and a shower room. We are going to consult an architect but I’d appreciate any advice and experiences from anyone who has already done this. Is it a worthwhile project?

If you intend to add a shower room then you will need planning permission. If it's just an office then you can convert it under Permitted Development (don't let anyone tell you it is Change of Use, it isn't, a domestic garage is still 'domestic' use class.) You probably won't need building regs for an office either as long as it's built of substantially non-combustible materials, and the electrics are signed off by a competent person (it's like adding a garden office.) But as soon as you add a toilet, it becomes classed as possible sleeping accommodation and will need building regs approval.
I am intending to do this myself in the near future. Very tempted to turn it into a proper annexe (and make some money renting it out to tourists) but the primary intention is for an office so will probably go down that route to avoid needing planning and regs.

Just because it has a shower does not mean planning permission will be required. It may require Building Regs.

Not all conversions are allowed under permitted development. PP is required if the use of the garage is restricted by way of a condition on the original PP for the garage (or house if built at the same time). This is quite common in houses and/or garages built from the mid-70s onwards. Conditions were imposed requiring that the garage be used for garage purposes only. An application would need to be made to relax that condition. Yu will need to check whether the garage was built under a specific PP, and if so, check that PP to see the conditions.

@wineymummy you best check the status of your garage before you start work in case it is covered by a condition such as the one I refer to. If you are proposing to use it as separate accommodation, PP will more than likely be required.