I went through several boxes of space heaters that looked like potential kotatsu heaters, opening the box to read the user manual. Even for the tiny ceramic heater, which is just eclipsed by a box of Kleenex in size, is inappropriate for kotatsu usage. Minimum space clearance - the space you should have free round these types of heaters - is three feet front and back. Unless you happen to be using a dining table as a kotatsu, please avoid these types of space heaters. The smallest space they will safely heat is a room.

Alternatively, you could use no heater at all. Since the comforter creates an enclosed space, it will already trap heat generated by your body. This is also a good way to save on electricity.

12/12/07 Update: From discussions with readers, it seems that the best alternative so far is the Cozy Legs Radiant Heat Panel from Indus-Tools.

Dimensions: 22.5″ x 16″ x 1″
Weight: 8 pounds
Why it would be ideal: “Radiant heaters emit infrared radiation that directly warms the objects in front of the heaters (rather than the surrounding air). If you only need heat by a desk or in a small section of a room, a radiant heater is quiet and will use very little power.” (pulled from site description)

Using triangular and elbow corner brackets, you could build a support for it by attaching these brackets into the table legs. Using triangular corner brackets would give it the most support, but having two elbow corner brackets would allow you to slide the panel in and out, according to the season.
Want to read more?
Project Kotatsu Post #1 - Introduction
Project Kotatsu Post #2 - Construction speculations
Project Kotatsu Post #3 - About heating alternatives

Dimensions
Dimensions for kotatsu I’ve seen online vary little. I prefer a rectangular table over a square table so I can stretch out my legs. On average, a rectangular kotatsu sold online measures:

• Length: 41-42 inches
• Width: 29.5-31 inches
• Height: 14-17 inches

That being said, you should place a priority on what works best for you, especially when it comes to the height of the kotatsu. If you have longer legs, then clearly you want something higher than 14 inches.

Materials: Table top and table frame
As exhibited by the photo below, kotatsu are made of four major components: blanket, heating unit, table top/tabletop, and table frame. Blanket is not an issue. For the last two items, I was originally thinking of going to the thrift store to cannibalize a table. However, most thrift store furniture is rather cumbersome and solidly built. The legs of a coffee table might serve nicely, though.

I could go to Menards or Home Depot for some lumber but that would also require sanding and finishing, which I’m not keen on doing. So what other cost-effective, suitable alternatives do I have? Hello, IKEA.

Table-wise, coffee tables would work the best, what with their already low height. A bummer I no longer live close to one. However, IKEA does offer some product through their website, which allows me to browse at leisure and wait until I have a weekend to drive down to Schaumburg.

IKEA’s Liden coffee table is nearly perfect and reasonable at forty dollars but is a rather narrow twenty-four inches across. Turning to the Make-Your-Own-Table section, I find a Vikon Amon table top for twenty dollars. Best of all, it comes in various dimensions: 47.25″ x 29.5,” 59″x29,” and so on.

It’s made of particleboard though, so I wonder if it’ll be heavy enough to keep the blanket from moving. Looking at various photos through Flickr, the table tops seem pretty weighty. Regardless, a table frame could be created by either creating a complete table with another Vikon Amon and some legs, or creating a table frame with an open grid with sanded lumber would work, as that would be hidden from view by a heavy blanket.

Materials: Heating unit
A proper kotatsu heating unit will cost about two hundred dollars. I don’t want to spend that much money, nor should I have to if I’m clever about it. Most heat bulbs are conical and wide across the top. Positioned sideways and covered with a wire hood or grill, this would take up more space underneath the table than I would like.

Inspired by heating cables for reptile cages, I found some heating cables by EasyHeat sold at Menards in various lengths for under thirty dollars. Originally for preventing metal and plastic pipes from freezing in the winter, I think it would also work underneath a table. Zero-clearance heating units are important here; if I can wrap heating cables round pipes without worrying for fire then heating cables are my best option.

The image below shows heating cables used to warm floorboards. This is much like the traditional Korean underfloor heating system, known as ondol (온돌). I happened to experience ondol when I was in Seoul and liked it very much. Sleeping on the floor was comfortably warm. Returning to my point however, applying the heating cables in such a manner over the entire table underside is possible and possibly even warmer.

How I would attach the cables in a sturdy manner is another question to consider.

A second option for a heating unit would be a heating mat. I found a small space heating mat through a beer brewery supply site for thirty dollars. I did notice other types of heatering mats for pets and feet, but they require pressure to activate. Only 9″x12″ with a long power cord. Ideally, this would go on the floor underneath the kotatsu as opposed to a heating unit on top. Since legs are on the floor this would make more sense. However, a drawback is that it only warms up to sixty degrees Farenheight. Then again, with such a small space to heat I’m sure it’d be enough especially when you add legs and such.

Conclusion (for now)

So my research has come up with a lot of different and interesting finds for building a kotatsu at home. DIY! My knowledge of electricity and wiring are still lacking so I need to read up on that. Setting the house afire or electrocuting friends are not goals of Project Kotatsu

Want to read more?
Project Kotatsu Post #1 - Introduction
Project Kotatsu Post #2 - Construction speculations
Project Kotatsu Post #3 - About heating alternatives