First off, a distinction needs to be made between value and marketability.
Value is how much the property is worth taking into account the type of accommodation it offers, the physical attributes, i.e. number of bedrooms, state of the kitchen, size, etc. While marketability is the readiness of the property to be sold. This speaks to the attractiveness of the property at the time.
Value...
The value of your property is to a large extent determined by supply, demand and personal preferences. While renovations and alterations can change the accommodation, this does not necessarily translate into an increase in the value of the property.
If you are renovating with the view of selling, you should pay careful consideration to what the market is willing to pay for the property on completion, and if the renovations or alterations are worthwhile.
As an example; installing a new kitchen will typically not increase the value of the property equivalent to the cost. Improving the property beyond the norm for the area could potentially limit the saleability as buyers generally are reluctant to pay more than a specific amount in the area. This is commonly referred to as over capitalisation.
Marketability...
Marketability is how ready the property is to sell. If the marketability is improved, you are likely to attract more buyers and get a slightly higher price. If the property is clean and neat potential buyers are more likely to find it appealing. A well-maintained and laid out garden can increase the marketability of a property, but would typically not increase the value by much. Many properties are only listed in spring when gardens look at their best. A coat of paint would in most circumstances not increase the value of the property; however it may influence the marketability despite the physical attributes remaining the same.
It is advisable to keep the house furnished, but not cluttered when showing the property. An empty house can make it very difficult for a potential buyer to estimate the size or visualise living in the property. Good estate agents will give advice and guidance on decorating to make the property more attractive for show days.