Like I said, experience in the area, for many many years.
Without outing, I myself am both a designer and in survey and planning, and family members are the architects, builders and trades.
We span across London / Home Counties and in areas where COL is much less.
Both @Yellownotblue and @Eastangular2000 are correct, though it appears different viewpoints.
Sorry East, it’s not “bullshit” or “saw them coming”, that’s simply what Yellow wanted and was happy to pay.
Yellow is correct that costs can, and do, spiral. But, respectfully, a subset of people on MN is just that.
Not everyone lowballs or has costs unexpectedly go through the roof.
Some do.
There will, however, ALWAYS be some level of unexpected extra costs, it’s completely unavoidable.
Usually over-run timescales as well, though that’s a different topic.
There’s also a lot of variables we don’t know here eg. location of property and personal views on brands, product styles etc.
Could you do a homes under the hammer jobby with £5 psqm carpet and have it look shit? Yes.
Could you go all out and spend your fortunes? Yes.
That could equally look shit FWIW, if it doesn’t suit the character of the surrounding area or style of the property.
So many variables.
To get an accurate quote for building works and renovations get numerous firms in. Bare minimum 3 quotes, more ideally.
Some can do everything “in house” as it were, some will do only parts of what you need and it can get complicated logistically marrying up the different trades.
Half you call for estimates won’t turn up.
You’ll get some out-price you as they don’t want to do it, but can’t say that.
Some will be cheap. That’s never good.
If someone has a short / no waiting list, it’s because they are crap.
”Mates Rates” / family can be problematic.
You'll get a massive price range, with tens of Ks difference in prices.
Ask neighbours and friends who may have had work done so you can nose at standard and style of work etc.
Price ultimately depends on what level and quality of products you want, scope of building works and materials needed, how much, if any, work you can, or want, to do yourself, the area you live in etc.
FWIW Yellows kitchen clearly has stone / solid surface worktops and for 9k appliances in the Neff / mid range is going to include a specific set of items.
25k of cabinets is a large size and not going to be of the flat-packed variety.
This is why it’s those sort of prices, absolutely not unusual.
You could go way higher with the likes of Wolf, Subzero, Miele, bespoke cabinets.
You could do for much less with your Hotpoint, Indesit, CDA, laminate worktops, flat-packed.
You could have own brand paint, or Farrow & Ball.
Ceramic tiles, or natural stone.
You get the idea.
OP, first start with the bits you can’t control because you NEED the professionals - build costs, architects, surveyors etc.
The rest can be negotiated / compromised accordingly as they are wants / nice to haves.
Hope this helps a little.