Every single answer above is correct.
I guess you don´t have multiple off camera speedlights, correct?
I´d consider different approaches and take the image that best works for you. Of course it depends a little on your budget, your time and with it on the money you can make renting it. The better your images, the bigger the chance you can rent for a higher price and/or to get more customers/sell quicker and therefore get the investment back.
Is the view out of the windows important to potential clients because it is a nice view, I´d switch on the lights indoor, and wait for the evening when the ambient light outside is low and matches the light indoors. The difficulty about this technique is white balance. The light outside will be very blue. If you don´t like that, you can correct it in two ways: the cheaper way is to get some smaller blue gel filter and cover the indoor lights, cutting them into pieces and sticking it on the lamps The more expensive would be to get a roll of orange gel filter (
see these from B&H) and cover the windows from the outside. It´s not quite easy to decide which filter you need, but I´d go for a rather deep orange - even if the color is too deep, it rather looks like a sunset. This is an approach they use for movies btw, but it works really well.
Second option for capturing the nice view: wait until around 1 hour before sunset - the light outside will be more yellow, but not as bright as during daylight (hopefully your window is not oriented towards the west, and you get light falling in). Put the camera on a tripod, set the focus to manual after you focussed and best use a cable release or something wireless - not touching the camera (and setting the focus to manual) is better to have the images perfectly aligned. Use off camera flash and take multiple exposures, each with the flash lighting at a different part of the room. Then you go to photoshop, open all images in one file as layers, auto align the layers and carefully mask out the darker areas of each image, revealing the flashlight for the different parts of the room. You might have to work with different brush opacities to blend them all together nicely.
Third option for capturing the nice view: HDR as suggested above - don´t overdo the HDR look though and keep it natural.
If the view is not that nice which is valid for the majority of the rooms rented, you don´t want to show it. So I´d overexpose the windows. Then you can still go with HDR and leave the windows overexposed, but a single shot might also work (even without flash to fill the darker parts). Using this "technique" will also make darker rooms look much more friendly and bright. Don´t be afraid to totally overexpose the windows and even get some lens flare.
Other things to consider: you definitely want to shoot RAW and edit later in photoshop and/or lightroom (or similar software) to brighten the shadows and lower the highlights for non-HDRs. Don´t be afraid of noise when bringing up the shadows - most potential clients are no photographers and wouldn´t care at all - especially for the smaller web images.
Set the aperture to around f 8-11 and choose the right shutter speed to get a correct exposure (with a good tripod or a bad one with using an additional cable release you can choose any shutter speed you want).
Regarding the focal length: wide lenses will make a room appear bigger which in my experience is what attracts customers. Architects tend to like longer focal lengths to present their work in a more artistical way, showing the details. Especially for wideangles: keep your camera level in every direction whenever possible to avoid perspective distortion. Having said all that, you might want to rent a wider lens. That 18mm you have equals 29mm on full frame. I´d go for somewhere around 10-12mm,
like this lens which is quite a bargain depending on where you live you will probably find a store that rents lenses.
There are even more ways to achieve great images, but these are the ones which are most cost effective and not too difficult to achieve for people with not the biggest experience in architecture photography.