Onsen in hotels; can non-guests use them?

I am wondering if non-guests can pay to use onsen which are in hotels?

I know some allow this, as I have done it before, but is it quite common?

I am planning a day trip to a small onsen town near where I live. I can find lots of information about hotels in the town, but not whether their onsens are open to the public. Do you think it's likely that at least 1 or 2 of the 10 or so hotels I know of with onsen in that town will allow non-guests?

Thanks!

It is quite common. Which onsen is this?

Most hotels allow a certain time period for non-hotel guests to use their onsen, especially in onsen towns. My advice is to go to the tourist information center when you go, and see the times for all the hotels. They may be similar, but there will be some differences, so you should be able to find one to accommodate you no problem.

Famous onsen towns like Kusatsu even have a pass you can buy that grants you access to all the open hotel onsen, and you can get stamps at each to show you've been there. Collectors like these books, and try to fill them with every onsen in the area. Whether the place you're going to is like that or not, I'm not sure, but it is quite common and I suspect you'll have no problems.

I know some allow this, as I have done it before, but is it quite common?

Yes, it is generally very common, although not all of the ryokan/hotels do it. But there is a small number of onsen resorts where it is less common.

Some offer a resting room and/or a lunch package, of course for a high cost. But most of them are indoor bath/family bath, not roten outdoor.

"quite common" or not depends on location just most onsen area's usual
urban big city no onsen area's not usual just other day services instead.

Find 日帰り温泉(higaeri-onsen)/日帰り入浴(-nyuyoku)[day trip spa]
from JP page menu of Hotel/Ryokan web if no Eng menu or not like this.
http://www.okunikko-kougenhotel.com/dayspa.shtml
Only bathing fee around 500-1000Y no need booking usual.

Also a lot of Hotel/Ryokan named with -plan[プラン(same sound)] provided
as lunch/short break/short sleeping pack plus need booking until before day like this.
http://www.tachibanaya.jp/dayplan/
貸切[kashikiri]温泉/風呂(furo/buro)/部屋(heya)風呂 is private bath.
和(食)[wa(syoku)]/洋(食)[yo]/中華[chuuka] are food menu Japanese,western and chinese.

Thanks for your useful information everyone. Sorry for the late reply.

I'm not really looking for a plan or any food, just the bathing!

In a guide book I have I've seen this '立寄入浴', which means 'stop-off bathing', am I correct? Would this refer to just the bathing, no plan etc?