The issue is the building work hasn't added space, you aren't going to add value by removing a wall or converting a garage into a study. By doing these things you hopefully make the house appeal to more buyers but there is also the risk that you change the house from something that lots of people are looking for into something less people are looking for. We once viewed a house where they had knocked down the garage to build a patio area. They told us as we viewed the house they did so because "no one needs a garage, everyone we know gets a guy in for odd jobs" - that might be so for them but not us. A full sized garage was high up on our lists of "needs"
Updating decor again doesn't add value, it expected as a given and the description in the 2018 advert states "immaculate 3 bed semi" as does the current listing so I wouldn't expect an increase in value now having done the decor. If it was in that bad state of repair how come it's still got the same kitchen and bathroom?
When you aren't getting as much as you thought for a house there are some awkward questions to ask yourself -
- Did we pay more than the market value and that's why there isn't the expected % difference between now and then?
- Did we spend more than the house has gone up making it work for us?
- Have we inadvertently made it less desirable?
Final point would in things hidden in the car - you mention a dog. Most of the house we viewed where the owner has a dog, you can smell them all through the house and it's off putting.