So I purchased my first home recently. One of my considerations was finding somewhere that I could fit a small sleep out on - they pay for themselves after a few years, increase your rental income thus helping you pay your mortgage off sooner and an extra person in the house brings down fixed costs like internet and power.
I looked at a few on trademe, but it was hard to find one that was the largest size (10sqm) allowable without resource consent. You can get a second hand one for between $6000 and $9000 depending on specs, but I found a guy who builds them. I drew up some draft plans for what I wanted and he's turned that into an actual building for me, total cost was $12,000 including double glazed window and ranch slider, electricity, fittings, insulation etc. When it's done, it's probably going to be the warmest and nicest room in the house.
Anyone else looking at portable buildings as a way to add value to your house?