Parents' house deeds

[Housedeedquery]

DM and DF are looking into adding me to their house deeds (only child). They haven't said much to me about it and I'm not very savvy with things like this so wanted to ask if there were any pitfalls/anything I or they needed to be aware of?

Have a mortgage on my own home and am married with kids. Would it cause any issues if I moved house/had to remortgage etc?

Don't know about mortgaging or moving.
But say your (no doubt wonderful) DH had a bash on the head, a change of personality, and ran off with someone. If you were a part owner of your parents' house it could affect the divorce settlement.
If their house was sold, they wouldn't have to pay CGT on any profit as it was their main residence - but this wouldn't be true for you.
If they both had to go into care and the LA were trying to recover care costs from the sale of the house, they might query if the deeds move was trying to avoid paying care costs.

Conversely, if Mr Jones dies, leaves everything to Mrs Jones, who dies and then leaves the lot to Daughter Smith (neé Jones), the estate would have to be over a million before IHT was due, see www.gov.uk/inheritance-tax

So I wonder what they hope to achieve?

Exactly as above happened to me. My sibling and l own 25% of parents house (my Dad's 50% was split between us when he died).

XH then left and had a claim on my share. Was very very messy

I also had issues with getting a mortgage as l was "responsible" for another property... even though it's mortgage free.

I'm also going to be screwed when l get moved over to Universal Credits

Why do they want to do this. It doesn’t seem to have an upside

It can get very complicated and is rarely advised so I hope they have taken legal advice - and maybe you should too.
If they are going to do this you need to be able to have some very honest and open conversations.
They would be very prudent to give you LPOA and ensure their Wills are up to date.

It will impact your tax situation. Owning second home and capital gains etc