Converting garage to garden room/office

[Housegouse]

We are planning to convert our brick built detached garage to office/garden room. Our house is already 4 bed house with two lounge. We have one DC and we both work from home. We are planning for other child in future. So, both DC will be given a room of their own. Third room will be out bedroom and fourth is currently my office room. One of the lounge is now a playroom.
We have been quoted around £5900 to convert the garage to office room. We are tempted to get that done due to the good quote we have got but not sure about the usability of that space yet.
We already have driveway in front with parking for 3 cars. So, we definitely don't need the garage to park car.
Do you think it's a good idea to convert garage to office room and is the quote price for the job looking reasonable?
Thanks

Forgot to mention garage is 5.5*2.5 metres.

That sounds really reasonable. Does it already have power?

It does have power connection and one socket already but we are getting the whole garage insulated, plastered with new sockets and downlights

That sounds v reasonable and yes, it's a good idea. Very few people want to garage their cars nowadays. Do you have a shed?

We will need to add a new shed in the garden. We will need to factor that price in alongside building the base cost.

We have exactly this done in our lovely rented house - ours has the heating connected to the house too (so radiators) plus veluxes, tons of power points, patio doors and built in storage and wood floors- I think that's cheap, so would be careful what it covers- ours would be good enough asa bedroom for instance. It adds lots of value if done well.

Good point about heating! Does your quote include that?

@Clymene we will be putting an electric radiator in the room.

@Crikeyalmighty would you mind me asking how much did it cost you?