According to wikipedia, "A kotatsu, used almost exclusively in Japan, is a low, wooden table frame covered by a futon, or heavy blanket, upon which a table top sits. Underneath is a heat source, often built into the table itself." - source

This instructable will teach you how you can build one for yourself for those cold winter months.

Step 1: Materials Check

Required:
- Ikea LACK coffee table (Ikea )

The legs on traditional kotatsu are 36cm (about 14.5") tall. When I was in Japan, I found this to be way too short for me to sit comfortably which is why the Ikea LACK works so well. The LACK also has a lower shelf portion which can be used as the final tabletop.
This can also be already assembled or unassembled. It can also be whichever size and color you feel fits your home. I used the bigger size.

- Kotatsu heater

The hardest piece of equipment to obtain. Try BeNippon or Ebay.
IMPORTANT: Kotatsu heaters are designed to be mounted upside down and have no required clearance. Do not use any heating element but a kotatsu heater otherwise you run the risk of starting a fire!

- A 120 V to 100 V step-down voltage transformer

The second hardest piece of equipment to obtain. American outlets put out 120 V but Japanese appliances run on 100 V. Try searching Google, Ebay, and Amazon. This is the one I purchased from Amazon but is no longer in stock. Look at your heater and see how much wattage is required to run it. You'll want your transformer to be able match that.

- 4 angle/L brackets

Get these at any hardware store.
IMPORTANT: Make sure that the screws that came with your kotatsu heater fit the bracket AND that the sides of the brackets are long enough to accommodate the thickness of the heater unit.

- Screws or nails that fit your angle brackets

These will be used to attach the brackets to the bottom of the LACK.
Just take a look at your brackets and get the right number and size of screws or nails to properly secure the bracket to the table. I used screws.

- Comforter/blanket thing

Based on the size of LACK that you have, you will need an appropriate sized comforter/quilt/blanket. It needs to be large enough to fit over the top of the table and have some slack to cover the people sitting under it.
Thickness depends on how warm you want it to be.
As far as design goes, feel free to go wild. It can be in any print you can find that will match your home or you can even make it yourself!

Optional:
- Electric drill and proper drill bits/screwdriver tips

If you end up using screws, this will make drilling pilot holes and screwing in the screws much easier.

- 4 Washers

For securing the kotatsu heater to the brackets.

- Tabletop

If you don't like the LACK shelf as a tabletop, you want a different color, or perhaps you want one that fits EXACTLY, getting a custom tabletop may interest you.

Step 2: Attach Brackets to Kotatsu Heater

Doing this step first will eliminate most of the time you would have wasted by measuring the size of your heater, placement of your brackets, location of holes, etc. etc.

1. Figure out the orientation you want for your brackets.
2. Line each bracket up against the screw holes of the heater and screw them in.

If you had opted for washers, be sure to place them appropriately.

Easy!

Step 3: Determine Placement of Heater

1. If your LACK is already assembled, remove the bottom shelf and flip your Ikea LACK over. Otherwise, just flip the table top over.
2. Place your heater + brackets on the underside of the tabletop.
3. Perform a dry test to make sure that bracket properly connects to the underside of the tabletop.
4. The location of your home's wall sockets will determine the heater's orientation. Play around too see which configuration works best for you.

Step 4: Attach Heater to Underside of Tabletop

Now that you've determined where you want your heater and which direction it will face, it is time to make it permanent. How you do that will depend on whether you chose to go with screws or nails.

Note: The number of screws/nails you will need will be dependent on the brackets you are able to find .The ones I chose have two openings on each side so I needed 8 screws total..

Screws
1. Grab your electric drill and drill a pilot hole through an opening on the first bracket.
2. On your drill, switch out the drill bit and insert the appropriate screwdriver tip.
3. Drive the screw in.
4. Repeat steps 1-3 for each bracket. I recommend doing it this way (instead of drilling all the pilot holes first and then driving all the screws in) because it gives you a slightly larger margin of error in case you are imprecise with your pilot holes or driving.

Step 5: The Fun Part - Decorating and Final Assembly

Now that your heater unit is properly attached, you may now flip your LACK right side-up again. Or, if your LACK is not yet assembled, now is a good time to assemble it. Make sure you don't put in the little pegs for the shelf since the shelf will be acting as our NEW tabletop.

1. Throw your comforter/blanket over the top of the LACK and then place the tabletop over the comforter.
2. Plug your transformer into the wall (making sure that the input voltage is correct)
3. Plug the kotatsu heater power cord into the transformer
4. Plug the other end of the power cord into the heater itself.
5. Turn on the heater!

Step 6: Enjoy Your Creation!

Take a seat underneath your new kotatsu and enjoy the warmth!

Activity suggestions:
- Watch a movie with your boyfriend/girlfriend.
- Have friends over for shabu shabu
- Light up a hookah