You can put together the most by-the-book yet fashionable suit, and is still won't be appropriate for your workplace. I wore the same three-piece suits I did at the Pentagon to my subsequent job. A couple days in, however, my new boss came to me and asked me to dress down to avoid setting a new precedent. It was several years of business casual apparel after that. Only after moving to a new job after several years did I return to the world of suits.
The goal of this chapter is to provide some brief introductions to what you might expect in your future workplace. It is not exhaustive, which would require many pages to cover every potential employer. Keep in mind that these are general guidelines based on observations that I've made over the last decade. The single best source of information for how to dress will come from the people who currently work there. Your boss will be able to provide the expectations, and your co-workers will help translate what those expectations mean in practice.
No matter what the job is, you should always have the means to dress a little better on short notice in case of an external facing meeting. Even formal environments tend to dress down day-to-day compared to entertaining outside visitors. Doing so can be as simple as keeping a jacket and tie on-hand in your office or cubicle it. The more public facing your job is, the more likely you're going to have to wear more complete suits on a regular basis and vice versa.
The applicable dress codes in the federal government are on a sliding scale of formal to casual. Some agencies are so large that the dress code will vary substantially by component. The Department of State lies on the most extreme formal end. Those working there have limited options based on a highly rigid dress code, and while you probably won't find it in the foreign service manual, it is enforced by a culture of bullying. The next notch over includes the sartorially conservative but somewhat more flexible agencies like Treasury, Justice, and Defense; the Hill falls on this part of the spectrum as well. There are more options at these places, but what you're going to see is the limited color palette of standard suits with a few glimmers of individuality here and there. A general rule of thumb is that agencies with a science mission have a more casual dress code for the average worker. A corollary is that political appointees are almost always going to be dressing more formally, even in those environments. The old adage of dressing for the job you want doesn't really apply in this scenario; dress for the job you have in order not to stand out in a bad way. It doesn't make sense for an intern to be wearing a three-piece suit with contrasting collar and French cuffs on their first day of work.
DC may be a company town, but there are plenty of other employers beyond the government. The myriad organizations, businesses that support the government, and lobbyists can have their own sartorial rules and guidelines. The same premise applies: see what the expectations are before you get there. As with the government, leadership will most likely be wearing more complete suiting attire more regularly, because they'll be interacting with people from other organizations more frequently than you will.
NGOs and Non-Profits. My first job in DC was at a non-governmental organization (NGO). Because it was science and technology focused, the vibe was mostly casual. About the most formal we had to be in the office was a button-down shirt and pants that aren’t jeans. I’d wager that’s a good baseline for many NGOs. I only had to keep a jacket on hand for important meetings. Enjoy your time here, because you will be able to prioritize comfort.
Contractors. I found that in agencies with substantial contractor populations the contractors tend to mimic the feds. The result is a more homogeneous look to the population, although close inspection of badges will reveal who is who.
Consultants. Contractors who don't live with you are called consultants. These folks generally dress more formally because they're trying to impress their clients and prove that they have spent their money wisely on professional people who are doing professional work professionally. I'm not really sure how they dress when they're not being seen.
Trade Groups. You may expect those representing business interests to be rather stuffy in their dress. My experience is the opposite. I've seen a greater variety of suits, ties, and ensembles from people representing trade groups than in any other DC population. I'm not sure why this is, but if you find yourself in that situation, embrace it and enjoy the extra freedom.
Business. You’ll often have meetings with people flying-in to represent their particular business interests. What you get clothing-wise is very much a mixed bag. I've seen everything from very dressed down folks, to those mimicking a more formal style, to regional affects like bolo ties. Keep in mind that the suiting culture is much less formal in most other parts of the country, so you'll likely see people don’t often wear a suit and their region may not have custom clothiers. It’s nothing to poke fun at, and it’s probably best not to comment on their suits at all.