Sesame, I used to have a granny flat-type office separate from the main house, so I can empathise. What do you intend to do in the office? It's a different thing if it's just to pay bills, vs studying, vs work on client projects all day, ie how long do you spend there? (the lack of windows is a concern). You might want to think in terms of zones: eg, a reclining chair to catch up on reading, separate from the desk, separate from the filing area (ie filing cabinets acting as divider). Maybe one wall painted with whiteboard paint. A little refreshment station (kettle/urn) to make a cup of tea might be welcome if the office is far from the kitchen.

Thanks for the ideas, the storeroom is based inside a commercial building. I intend to spend around 10 to 12 hours a day working here. My concern is that I want to create pleasant work space so that I enjoy working. Also not having any windows is a bit of a worry. I see all these beautiful transformations in Houzz however I don't have a clue on how to go about and create a good work space.

Sesame, given you will be spending a lot of time in the office, then the practical considerations such as fresh air need to be addressed. I once had a naturopath swear by ionisers to freshen up the air, you might want to check that out. You could think about a large painting or photo to create a "view". There are also stick on wall decorations that may do this too. Google "wall art", "wall decal" for ideas. Good luck.

Thanks so much for the ideas, much appreciated.

Have you thought of ergonomic office chairs? They are great for posture improvement. you seem to be starting from scratch with office design. I would definitely recommend getting comfy furniture

I would put the filing at the back. Desk near door with a rug under to create the desk zone, warmth and softness under foot. Keep to bright colours on walls and white or pastel blue sky ceiling. Greys and beige to be avoided as will suck the light out. You could wallpaper a scene on one wall. Fan and ionizer.

I would cover one wall with a wallpaper sticker of someamazing outdoor scene. They are brilliant for lifting the spirits.

I've used thesein office space and they look brilliant:

https://www.eazywallz.com/apps/customizer?id=422280341

I'am wondering if it would be legal to use this space as an office without any windows or ventilation, fire escape signs ?