Hi we have a 6 year old red cocker spaniel who is resource guarding. It started when he was around 8 months old but it has only been my handbag and grocery bags when we have brought the shopping in until now. It started getting worse just before Christmas as he began guarding my son's clothes and other items in his bedroom. My son was in a car accident right before chesters behaviour started to get worse his girlfriend was badly hurt and he spent a lot of time in his room quite depressed with chester with him. Could this have set it off? He has bitten a few times when we have tried to get the item from him and my son cannot get things from him either. We got a behaviourist in but it hasn't helped as they havnt given us any training for when he is actually in guarding mode. He can sometimes start growling for no reason when we stroke him or if we dry his legs after a walk. When he is not guarding he seems totally fine and loveable. It's so difficult as it's always our belongings he guards. He can guard food but it's only things he shouldn't have e.g chocolate he never guards his food bowl or his toys. Sorry for the long post we are really struggling

With any new behaviours, a vet check is always recommended.

Definitely vet check if it has gotten worse recently.  The growling when you stroke or dry him suggests that he may be in pain somewhere, which is common with issues of aggression.  I would also highly recommend a full thyroid panel, as cockers are prone to hypothyroidism and this can also cause or exacerbate such issues.

As for what to do in the meantime...

1) Management: try to keep anything that he might steal out of his reach.  Don't allow him in for a short time in if there is going to be stuff around for him to start guarding (e.g. the grocery bags).
2) Your reaction: if he does steal something, don't make a big deal out of it.  Act as if it doesn't matter.  The more fuss you make, the more stress, the more he will guard.
3) Swap.  Both in practice sessions when he is not guarding - swapping a low value item for a higher value one, repeatedly until he is happy to give up the lower value item.  And in real life situations - offer something fantastic in exchange.
4) Work on a drop cue; I've posted a link at the bottom.  This should always be delivered in a neutral or happy tone, never in anger (see 5).
5) NEVER try to just take something from him, and NEVER act with any anger or confrontation.  These behaviours WILL make his worse.  Resource guarding is born of fear/anxiety (fear of losing an item) and stress, so these only add to that.

If he won't swap directly to begin with - very common, especially when it's gotten to the stage of aggression - walk away, let him calm down.  Call him away, if he'll come.  Distract him (but remember to still keep it casual!).  Get him into another room, doing something else, or into the garden, then go and fetch the item.  Do not try to do this until he's well out of the way and cannot see you (and shut the door!).

Alternatively, get some amazing treats, and lay a trail from him to another room, garden etc.  Again, wait until he's out of the way and cannot see you before you pick up the item.

Over time, if you make no fuss when he steals stuff and he gets amazing stuff for coming away from items, he will calm down and the guarding will reduce.  It doesn't usually take that long, providing you stick to it and never try to confront him or force him to give something up.  Of course you may have times in which you need to get something quickly (i.e. dangerous item) in which case do what you must, but to the best of your ability, don't let this happen in the first place!

Drop cue video:
https://www.youtube.com/watch?v=tVivnOwiMoA

Thankyou nikita for your advice. He has been to the vets and he has had a blood test done so his thyroid is ok. They had a bit of a query about his liver so we are booking him in for a liver function test. With the drop cue he doesn't actually pick any of the items up or steal them he just sits on them. I will definatley give the swapping a try although he can be quite scary growling I'm not sure how close he would let us get to be able to swap.

Different breed and one that's not known much for resource guarding, but I don't push her and have always offered a swap rather then try and take things off her just in case it sparked RS.
I discovered a treat she will sell her soul for which is dried fish cubes, if she knows they are available it will bring her from miles away. If she is reaching the end of a particularly favoured chew and I need to get it off her before she tries to swallow it whole, then I get some fish cubes, let her know I have them and chuck them away into another room no she will spit out whatever she has and charge off after them. Handy if they are well scattered as it gives me plenty of time to pick up whatever she had and put it away.
It's got to a point now that all I have to say is 'fish cubes' and she is off to the cupboard where they are.

> I will definatley give the swapping a try although he can be quite scary growling I'm not sure how close he would let us get to be able to swap.

In that case, calling away or laying a trail to move him would be your best bet (with you out of the way once you've laid it so he follows it).

Practice doing swaps when he's not guarding to start with, to get that going.