Far too expensive

I hope they fumigate the hotel and cottage rooms after pets have been there. That costs money.  My sister in law rented a house to a couple whose pets left the place infested with fleas. That certainly cost her money.

Do  you know, I've never once met a cottage owner or hotel receptionist that has ever mentioned "fumigation"! Did your sister in law not check with the owners that the dogs were up to date with flea/worming treatments and inoculations?  

The tenants did a moonlight flit after a few months of a six month tenancy - rent arrears not paid - traced overseas - and house infested with fleas. Not a happy lady.- and specified no pets after that. 

Are the rooms that allow dogs ever let out to non dog owners?  I do know that ferries have pet friendly rooms but I would assume that they are used permanently for that purpose.

I'm not sure, though I know that the last hotel we stayed at specified for non-dog owners that they had some rooms which were specifically "dog free". Many specify "pet friendly" rooms. 

Of course, in cottages, there are no such restrictions.

All this is getting a bit away from the OP, but I would just point out that many establishments  look on dog acceptance as a sort of loss leader - the lady who owned the cottage we stayed in in Derbyshire recently said that over 75% of her customers were dog owners - she couldn't afford to alienate them by over charging!

On longer routes the ferry companies set aside a few cabins specifically for passengers with animals. Those cabins have no carpets - just Lino flooring so that they can be mopped out with disinfectant daily. All the other cabins, lounges and restaurants are out of bounds to animals. Other passengers are not expected to sleep in cabins where animals have been. BrittanyFerries charges £35 per dog on routes to France and £50 to Spain.

As evident by the photos routinely shared on Facebook, dogs don't always stay on the floor. I presume that the mattresses are also sprayed.

Getting back to the OP, although we may have different views, I think for him/her the right decision has been made in deciding to stay elsewhere. 

To answer your last question, sometimes. It all depends on what we are looking for, what the basic price is, what kind of price is being charged for a dog. Some places we have used, hotels, they might provide a dog bed, blanket, treats, etc…Trips like this, we usually take a brand new quilt and quilt cover ourselves, plus anything else our dog might need, we don’t let them use anything provided, but it is appreciated. Have no issues with there being out of bounds areas for them, and if the hotel wants us to put our dog in car, rather than leaving in room while we have breakfast, we do this, or have room service. We have used Travelodge stopovers, and they definitely don’t have rooms put aside as dog only. Some hotels have a dog friendly eating area set aside. Our dogs aren’t allowed off the floor in any place we visit, and they are supervised at all times.

We look carefully at the price a site/CL might ask for regarding a dog, and what they might be providing for that price. The answer is sometimes not much beyond a bin and that being emptied! A few do have dedicated dog exercise areas, and are very pet friendly. If it was above a £1 and already on the expensive side, we would dismiss it. We have never used a dog wash on any site to be honest, they are surplus to our requirements, as 40 years with a bucket and a sponge has served us well, but others might find them useful. Some of the Club Sites are not that good for dogs, others are very good because of the location, say within a Country Park, or Estate. Don’t have any issue with CLs that don’t allow dogs, there will usually be a good reason, and it’s the owners who call the shots. 

Sadly ET, this isn’t uncommon across the property rental industry. I worked for a friend who had a portfolio of properties for let, many her own and some for clients. Trashing of properties, unauthorised pets, unpaid rent, moonlight flits are all par for the course. Even interviewing, references, deposits don’t mean much. My niece has a property down in Kent, numerous repairs, redecorating issues down the years. Never had to fumigate though.