In addition to your apartment rent, other costs are to be taken into your budget plan: water, gas, electricity, internet...
Normally internet is not included, so you will have to contract by yourself with internet companies, however we advise you to ask the real estate agent regarding an internet line already installed or not in the building.
Water might sometimes be included in the rent, but usually it is not, so you you will need to contract with companies not linked with your apartment real estate.
When making the contract, the real estate agent will let you know the water, gas and electricity companies number, but you will have to contract in your name with those companies. A precision, even when you do not use water, electricity or gas, a determined minimum amount is taken.
You should better not forget to count those cost in your budget before renting an apartment, but how much does it cost?
Difficult to answer to this question as those cost vary depending on people cutlures, habbits, and lifestyle.
To give you an image, I will present you my personnal consumption case as a 1 resident, living in a 24sq meter 1K, on the 10th floor of a mansion from 2006.
Of course I am sure you understand cost may vary depending on the orientation of the room, the sunshine, if it is a corner room or not... but I prefer to precise it one more time.
I consider myself as a lambda european person, trying to minimize her consumption for environmental reason, but I am not too strict and could clearly consume way less! I am using the air-condittionner in winter and summer when I am inside the room. I cut it when I am not in my apartment, and try to do not use it if I think I could do without it. (usually don't use it between March-June & September-November).
Due to the air-conditionner consumption, my electricity bill increase in summer, and especially in winter, but it never exceed ¥3.500 per month ! Even in winter, the most apropriate average might be ¥3.000.
Gas bill might also increase in winter because of hot water (bath or longer shower). But same with electricity, it usually do not exceed ¥3.500/month, whereas out of winter, it is closer than ¥3.000 or less.
Here is the average sheet of my utility fees cost:
・Water ¥2.500 - ¥3.000
Under ¥10.000 /month
In my apartment, internet is not included, so I subscribed to a box and a mobile phone in the same company. It cost me around ¥12.000/ month, mobile phone included, but I know it is possible to find cheaper, espacially for the mobile phone!
Mobile Phone ¥7.000
I hope those information might help you to plan you budget if you are thinking of renting an apartment in Japan, and do not forget that those cost vary depending on people and apartments.