Yes you could, BUT demerara sugar is less refined than granulated sugar and will therefore impart a molasses like taste and a brown colour to the product being baked when using it.
This is good in some cases (eg a fruit cake) but not desirable in others (eg a Victoria sandwich).
Brown sugar is your best option
Demerara sugar is unrefined, usually somewhat coarse but dry sugar. It is not brown sugar, which has clumps together. It is pale golden in color and can be called either raw sugar or turbinado sugar in the States. Caster sugar is a very fine grade granulated sugar, most often used for baking as it causes cakes to rise better, or for putting on fruits and berries as it melts faster. It is sometimes sold as berry sugar.I recently bought some demerara sugar which was ground up to caster grade. It worked very well and was delicious in a lemon cake.You can make your own caster sugar by whirling up granulated in a blender.
granulated sugar
You use granulated sugar unless otherwise noted.
Granulated sugar shouldn't be used as a substitute where powdered sugar is specified in a recipe; granulated sugar will be too coarse.
You could substitute regular granulated sugar instead. (Chapter 4. Pg 68 Professional Baking)
yes yes
You should be able to substitute granulated sugar doe castor sugar without any significant detriment to the resulting product.
Demerara Sugar
Confectioner's Sugar (powdered sugar) has a completely different consistency and quality than granulated sugar. You cannot substitute one for the other.
Not unless the cake recipe specifically calls for icing sugar. Granulated sugar has a specific weight to measured volume and will provide that specific quantity and sweetness to the cake. Icing sugar has a different weight to volume, different texture and a different level of sweetness. It is important to use the specific ingredient called for in the recipe being followed in order to obtain the proper result.
rwgular sugar is granulated sugar. the other kind is confectioner's sugar, or powdered sugar
Regular granulated sugar is used most often but some recipes also call for brown sugar.