I have done some of the work for a business plan before however I am not an expert on the subject.
Basically you need to calculate what your actual operating costs are. Take into consideration "everything" you spend to keep the doors open. Your annual rent, electric, heating, telephone, internet, trash collection, water, sewer, property taxes, building maintenance, fire alarm system and insurance costs should be added together and then divided by your business's total square footage. This will give you the annual cost per square foot just to have the open sign on the door.
Next you will need to figure the total compensation package (matching SS, insurances, benefits, payroll expenses, 401k contributions, etc.) for all "necessary" employees. These are the people that you MUST have to operate your business.
If you keep an inventory of parts to sell you will need to figure that separately from your operating expenses but include it in your business plan.
With the above information you know what it costs you to operate your business. Now you need to calculate your profit from each vehicle or job that you do. Using previous jobs try to estimate the material costs, shop supplies, and hours of labor necessary to complete the "average job." You may need to use your best and worst jobs based on biggest profit and smallest profit. (I wouldn't count jobs you lost money on.)
Once you have an average cost per job, estimate how many jobs you "plan" to do per year. (use your job labor estimate and divide by a 40 hour week to get number of weeks per job and then divide that into 52 weeks for the number of jobs per year)
Now you need to take your total operating expenses (building and employees) and divide that by the number of jobs per year and this will let you know how much money you have to make per job - just to break even. This number must be added to the cost of the job and then a percentage added to it for your profit margin.
Banks will not lend money to a business unless they are sure that business will be able to pay them back. So proving that you are actually making a profit on every job after deducting all your expenses is critical to your business plan.
You then write a plan that states your current production/sales rate and your annual profits. Usually you are going to the bank to get money to invest in equipment or real estate so your plan should let them see how purchasing these items will increase your profitability. Let's say you are doing 10 jobs per year and clearing $2000 per job. Your plan states that with the purchase of the needed items you will increase production to 13 jobs per year thus producing another $6000 per year profit showing that you will be able to repay the loan.
The reason for getting the cost per square foot on the building is to enable you some leeway in working with your numbers. You could always state a smaller square footage actually being used for production (the rest office or storage) to lower costs and increase profits.
