Accessory Dwelling Units (ADUs) are an affordable type of infill housing. They are small, include a bedroom, kitchen and bathroom, and can be either attached or detached from the main house on the property.
Modern-Shed, a company out of Seattle, creates small "sheds" that reinterpret the conventional meaning of such a structure. They can be used as yoga studios, art studios, and guest rooms, and combine the economy of a manufactured home with attractive design (at least I think so) to achieve a product with a lot of potential.
Here is a short video that really captures the spirit of what the ADU and small homes movement is all about:
They come in sizes from 8' x 10' up to 16' x 40', and prices range from $11,200 to $81,000.