Next Year budgetting exercise normally starts before the year-end. Previous months' actual Income and expenses are pro-rated for 12 months to find the actual figure.
For example:
prorated expenses = Last ten months actual expenses/10*12.
Like-wise Income also calculated based on Current Year Month-wise Income and gives a certain percentage of increase/decrease for the projected next year figure.
Next year budget figure depends on several factors like projected business growth (income) based on last few years trend, increase in employees salary plus new recruitment of employees, Office Rent, Transportation, Electricity & Water, office expenses, Insurance and so on.
Taking all these factors into consideration a certain percentage of increase/decrease in the prorated expenses is calculated for next year projection.