Cosmetic things are not doer-up... you are EXPECTED to change cosmetic things to your own taste when buying a house.
Doer-ups require 'modernization' so things like rewires, new kitchen, new bathroom, new boiler/heating system, insulation etc... these houses often look dated but its not the 'decor' that make them a doer up. They are habitable and homely just not up to current safety standards and not had updates in a long time.
If you are asking about houses that require actual work they say that it knocks approx. 15% off the value (+ the cost of works) to account for the drop in available market able able to buy a house that needs disruptive works doing.
EXAMPLE
So say a house was £159k value in good condition but is a doer up:
£159,000 - 15% (for market drop) = £135,150
known works needed:
Bathroom = £5,000
New boiler = £2,500
Double glazing = £6,600
New flat roof on small extension = £2,000
= £16,100
£135,150 - £16,100 = market value as a doer up approx. £119,050
You could chance your look starting the listing up to 10% higher at £130,955 but be aware people will offer under using the works or survey as reason.