OK, let’s put things in perspective here; the 16-35 IS L is a better lens than the 17-40 L
but it’s not within David’s current budget. The 17-40 is within budget and a used one in excellent condition can probably be bought for around £300 and there are plenty around to choose from. So, rather than just harp on about the 17-40 L not being as good as the 16-35, let's see some examples of how it might work in real world type usage for property photos.
Before anyone start critiquing the following photos they were taken
purely as a
quick test to check that my
second hand (but mint and boxed) 17-40 L worked OK. So do excuse the poor taking angle and the lack of any lighting to compensate the contrasty winter sunlight (if you look you can see the packaging the lens came in lying on the worktop just beyond the sink – the photos really were taken that quickly!).
The following pics are JPEGs taken straight from the camera (a 6D) without any tweaks in Photoshop, and were all taken hand-held at f/4, at between 1/30 and 1/40 of a second, so there’ll probably be a bit of camera shake there to influence the sharpness, so please bear that in mind
First off 17mm:

Then 20mm:
[URL='https://flic.kr/p/251PEu5']
Then a comparison from a 24-105 L

So there you go, some real-world examples with the ability to click on them and pixel peep. However, I doubt any estate agent or potential house purchaser is going to do that (just how big do photos display on the Rightmove website?!). So I can't imagine them saying "Those corners look a bit soft at f/4, the photographer obviously doesn't have a 16-35 IS L"! For a start, depth of field will take account of that anyway in most house interior shots, unless you're taking a photo a flat wall straight ahead, and how may houses are big enough for that?
I hope this is useful to the OP, sorry the composition of the photos isn't better, etc. Anyway, take your time deciding what's right for you and best of luck choosing.
[/URL]