Business plan is a sine qua non condition for starting a business. Any business. In many countries, you will need it if you want financial support from the state or credit from the Bank. They don't care if you are talking about a restaurant or an Internet advertising company. If you want credit, support, acknowledgement, you must explain the way you are planning to run your business and where it will go.
The translation business is no exception. Nor agencies, or freelance businesses. Of course, a freelancer has a different relationship with loans, banks and investors but, anyway, he / she is running a business and it is good to be clear about it. When you contact a company, offering your translation services, you are not asking for a job: you are making a business proposal, so, you are a partner, a services provider, not an employee. With all the good and bad consequences that may bring you.
So, if you have a business, you need to plan it. That's how the market works. But that is not the only reason to spend time working on a business plan. Actually, planning is something that saves time and, taking into account that translation is a business where a per - hour fee is often applied, you will be taking care of your money if you write a business plan.
That is a very complex document. You can run out of your patience sometimes. Financial plan? Risk analysis ? Come on! I am translator! I did not study Economy or Business Administration! Well, probably you didn't, and you will have to do something about it now. Any way, a business plan for a freelance business doesn't need to be so complex. Let's see what it should include.

Here is a very basic structure I suggest for this document:

Front page with the name of your business: It will be your own name in many freelance businesses but a logo and / or a slogan is always helpful. Make it short, easy to remember and appellative.
Executive Summary: It makes sense to write this part at the end and it should focus on objectives, services, target markets, financial plan.
Business idea and opportunity: You must convert it into a project. Why do you think this is a good way to make money and how will you prove it ?
Business objectives: Where will your business be in a year? In three? Define quantitative (financial and other) milestones. Be realistic but I recommend some optimism too. This is your business, define the goals that will make you happy and work towards them!
External Analysis: Economic indicators must be taken into account before starting a business. Define your target market. Are you delivering your services to the final client or are you in a B2B business? Include in your market research not only your target market and competitors, but also potential partners and providers. To convert the first ones into the second ones could be a win – win strategy.
Marketing Plan *: Main point. We will come back to it. Translation is, by definition, a global business. You can live in Siberia and have clients in Australia. You only need a good Internet connection. So, the Internet will play a main roll in your marketing strategies. But you can also use some other approaches. Define at the beginning how to reach your market the fastest. Some important points: Web page, on-line profiles, social networks and translation conferences.
Products and Services *: Products/ services summary, prices (rates) you are planning to charge. In the translation business, services are more often offered than products, take that in to consideration, they both are not sold the same way and should not be presented the same way. We will come back to prices a couple of paragraphs ahead.
SWOT Analysis (strengths, weaknesses, opportunities, threats): This is a nice qualitative tool. Be honest. Finding your weaknesses and the threats in the market will only help you.
HR Plan: Usually it is only yourself but it can also be a partnership with another translator. If so, define the work for every one. Be sure that your CV is written in all your target market languages and have several versions for different clients (agencies, direct clients, partner translators).
Financial plan *: Key point. It is the soul of the business plan. We will be coming back to it later when we talk about prices. Basically, you must find the way to make more money than what you spend on the business. That is your profit. And that must be enough to pay your bills and have the life you want to have. Otherwise, you should leave the business and find another occupation that gives you a better life. Right?
Risk analysis and contingency plan: What could actually happen that could collapse your business? How will you avoid/ minimize/ face it? Be realistic here too. It will help you when things go wrong and you will have no time to work them out spontaneously.
Appendices for more detail. Put your CV here as well as any other's involved in the business. It is also recommendable to include any certifications and professional degrees that qualify those involved in the business.
Now, I would like to explain a little more the three points above marked with *.
Marketing Plan is often forgotten at the beginning of the professional activity. However, it is fundamental for the business. Even very experienced and well established translators must look for new clients or they would be in risk of losing their incomes from one day to another. Sometimes you start the business because you have a very good client (or two). That is the moment when you say: “This is what I want to do and with these two clients I can earn money enough to pay my bills. Maybe even more than what I am earning now as an employee” But then, someday, your two clients change their minds about your services, or lose their business, or die. And you have no more incomes. So, write a list of marketing strategies and work on them a fixed number of hours per week. Even when you feel comfortable with the clients you already have.
You must exist on - line. You need a web site and a strong presence in social networks. But specially dedicate some time to networking with other translators. They are the main source of new jobs for many people in this profession.
Products and Services you will offer is something that you should think about before starting. Are you able to transcribe audio or video files? Are you a translator or an interpreter? Is Training included in your competences? Costumers need to know that you know what are you doing. Improvising is good but in a controlled way. Otherwise it could be taken as a non -professional behavior. Remember that your prestige is your main competitive advantage in a services industry.
How to charge your client? There are two important methods in translation business: per - word and per – hour. There are others, but these are the must used and they make sense because a word and a hour are very internationalized criteria. You will never need to explain to your customer what a word or a hour is. You will never have to discuss about it. So, you will apply a per word fee in this translation project...but which one?! How can you make a price respected by the market, interesting for the market and be happy with it?
So, you need a financial plan. Something that will help you to know how you will manage the money. You need to take into account not only direct office expenses (software and hardware, paper, transporting if you are an interpreter) but also promotional (expenses in conferences, memberships in professional associations, business cards); taxes and health insurance; services (accounting and legal advisors) and indirect costs (electricity, heating/ air conditioner). You will not work at weekends, at least not every one of them and probably you will not work 12 months a year and even more probably, one day, you will retire, so you have to practice a price policy according to all that and to work very hard to apply that policy.
This tool: http://www.asetrad.org/pdfs/CalPro_v1.3.xls (Spanish) can help you on this calculations.
Other practical tools (in English) can be found here: http://www.proz.com/translator-rates-calculator/ and here: http://freelanceswitch.com/rates/ (per hour method).
These three points, Marketing plan, Products and Services (prices) and Financial plan, are key points. It is important not only to convince third persons that you are serious about your business but also to help you to save time, effort and minimize loss throughout your business life. They are the main core of the Business plan.
You need to be sure about what are you offering, how you will sell it and how you will earn from it.