Whatever building structure you go for I highly recommend a combined heating aircon unit (pictures up thread) - I'm loving mine. It cost £1.2k including installation and is supposedly no more expensive to run than traditional electric heaters and it cools as well which will be very useful in the summer as my home office can turn into an oven. Its also extremely quiet so won't upset your neighbours. If I switch it on early in the morning or am still working in the evening I use the "outdoor unit low noise" function just in case.
This is my only significant concern about mine - summer and heat!
I have electric underfloor and I have to say, the building as a whole is outperforming the house in terms of heat loss overnight even with the very cold nights we're having. Looking at the data on the EUFH, it's coming on for a pre-heat of less than an hour most mornings - the controller is still learning and did manage some dramatic overshoots early on (hit 23 degrees on a 19 degree target).
But summer is concerning - the office is north-facing so gets most of it's direct sunshine in the mornings; so there will be mitigation in the form of making sure the blinds are closed to reduce solar gain in the morning etc.
I am not morally adverse to looking at AC (though I actually don't have a lot of wall space for the unit) but want to see how things perform over summer as I'd rather try and avoid adding to my already significant carbon footprint etc.!
Before it was furnished (and before blinds were fitted), we did get some very hot days and the sealed box went up to 35 degrees - I was pleased to see how quickly the temperature came down with airflow though and I am hopeful that I can keep the temperature down to start with.
Anyway - time will tell whether I made a misjudgement.
As for value to property, it wasn't a consideration for us, but it does ease the pain (though there are some slight accounting/tax challenges if we did sell as my company built the office for me so I really need to let the asset depreciate and then the company dispose of it at a 'fair value' but it's on a fairly long depreciation schedule (I actually don't know how long off the top of my head)).
We're about to do my wife's office next - again the joinery by Kerf - so we end up with a 2-bedroom cottage with 2 fitted home offices - kind of niche, but for a childless couple who run two businesses from home, it works for us!
@TimC - that's going to look bob on that - love the floor to roof apex window!
