When you need to add offices quickly, it might seem like you don't have a lot of options. However, the options you do have are quite versatile and can work for different businesses easily. What you need to consider is how close you need to be to a particular site, why you need the new offices, and whether you could possibly transform these offices into your new permanent ones, if you weren't planning on that already.
Mobile Bungalows
If you have a dedicated worksite (like a construction company property) and need your workers to be onsite, then you need something that can be plunked down on the property. If the need is temporary, like your regular offices need retrofitting work before anyone can go back in, a mobile-home type of structure, like a mobile bungalow, would work just fine. These are those rectangular units you often see spring up when buildings need so much work that offices have to be temporarily relocated. These come with air conditioning and heating, working windows and lights, and ample space. However, they are definitely meant to be temporary, despite their long-term use by many companies. When you walk in them, your footsteps transmit vibrations down through the entire structure, for example, which can get annoying after a while.
Renting Existing Space
If your workers don't need to be at the same site, you could try renting existing office space somewhere in the city. That's great for very temporary assignments but can become pricey as you deal with rent increases. Neighboring offices can create problems for yours through noise. You'd also have to deal with adjustment issues like new traffic patterns that could make employees late during the first few days.
Permanent Modular Construction
If you'll need space for additional employees or space for a very long-term relocation, try a permanent modular building and make that the new office space. Moves can be disruptive for employees and business, and if anyone is going to have to move to another space for a long time, you may as well see if you can make the space permanent and spare the person another move. Modular units are built to the same standards as your other buildings, but they are built in modules in a factory, so construction is rapid and efficient.
If you'd like to know more about modular construction, you can contact a manufacturer who can show you floor plans and discuss timing with you. You can talk with a service, like Avon Modular, for more information. Permanent modular construction can be up and running quickly, giving you enough time to plan the move and prepare for any disruptions, but fast enough so that you're not waiting months for the buildings.