Dungeon mastery: castle building...

DETERMINE OCCUPANTS 
"My home this is!"

Calculate who lives in the Castle: Generate a Quick list of NPCS and give them Social Ranks.

IMPORTANT PEOPLE: Lord, Lady, Children, Guests, Adult Children, Senior Officials

SOLDIERS: These are troops (other than senior officers).

SERVANTS: These are the Household Staff and the Menial labourers

Calculations
Personal Servants* = #people of Importance x SRM
Smithy & Stables* = soldiers/20
Kitchen Staff* = (Important People + Soldiers + Personal Servants + Smithy & Stables Workers)/20
*Round upwards

Social Rank Magnifier (SRM)
KING/DUKE x 3
MARQUIS/COUNT x 2
BARON/LORD x 1
KNIGHT x1/2

LIVING QUARTERS
"Trying to Sleep Here!"

LORD/LADY: Located in the Upper most levels, these are the quarters of the Top People in charge. This chamber is 400 - 500 square feet and 9-10 feet high.

GUESTS: These rooms are for adult children of the Ruler and Senior Officials. They are near the Upper most floors (but not likely on the same floor as the Lord/Lady's Bedchamber). 'Such Guest Rooms' should be 200-300 square feet.

CHILDREN: For reasons of security these are on the top floor. Such a Nursery should be 30 square feet. There is likely a Servants quarters and Guard post on this level.

SERVANTS & TROOPS: Crowded into Three Tier Bunk beds, they have about 20 square feet per person. Most likely this is a Servery (Quarters for Servants) or a Barracks (Troops Quarters).

UTILITY ROOMS
"Is that elf in the stew pot? It tastes like elf."

KITCHEN: There will be Work Tables, Sinks, a Hearth for cookery. 150 square feet + 1 square foot per person served. Most likely it is used to cook for Nobles, and servants and Soldiers alike if their places of residence are scattered. Else there might be a second kitchen attached to a Barracks or Servery. Some Kitchens also have a Separate larder for the hanging of Game (other than the Cellars).

BLACKSMITH'S WORKSHOP: This is the room with the Forge, Anvils, and hammers. They look after equipment; make repairs to tools and weapons, Horses, etc. This is 150 square feet + 5 square feet per soldier.

GATHERING ROOMS
"Bring on the Dancing Gnomes!"

THE GREAT HALL: This is a huge chamber with Chimney or fire pit, maybe a balcony for musicians, etc. Its dimensions are about 14 - 20 feet to ceiling. Area is 500 square feet + 50 square feet per person.

COMMON HALL: This is where Troops and Servants eat their meals or Guard Rooms. These should be 50 square feet + 20 square feet per person.

CHAPEL: A private Chapel might be as small as a Guest Room or you might be dealing with something the size of a Church. It should be about half the size of a Great Hall, with maybe a balcony if the ceiling is high enough - where the Nobles sit and watch the show. There is probably an altar. A Cleric might have an apartment connected to this chamber.

STORAGE AREAS
"Wow! That’s a lot of Swords..."

ARMORY: This is filled with Weapons and Armour. It is likely 1/3 the size of the Blacksmith's Workshop.

BUTTERY: This is where the Servants prepare Beer and Wine in jugs before taking them to the Great Hall. it is about 1/3 the size of the Kitchen. It may have a Cistern (or a Tap).

PANTRY: It is where Linen and Tableware is stored along with bread. This is about half the size of the great hall's kitchen.

CELLAR: This is where Stockpiles of supplies against famine and Sieges are stored. Barrels filled with Preserved meats, Ales, Wines, Salts, Spices, cheese, honey, dried fruits. All stored in a cool cellar for preservation. The Ceiling is about 15 feet high, and is 40 cubic feet per person for 6 months.

GRAINARY: This is a Dry-store filled with Barrels of Grain (or Sacks). it is about half the Size of the Cellar at 20 cubic feet per person per 6 months.

STABLE: This is 100 square feet per horse (larger for Monstrous Mounts +25 square feet per HD).

STOREHOUSE: Household Tools might be stored here. No Specific Size but assumes 25 square feet x SOCIAL RANK MAGNIFIER.

OTHER DETAILS
"Has anyone found the toilets yet?"

CISTERNS & WELLS: A thirty cubic foot Cistern can hold 250 gallons of water. This might mean a Cistern of 30 cubic feet of water per person per six months. This could be Storage Tunnels with 10 foot diameter barrels. A Well might simply be a vertical Shaft for the storage of Water with a lid. The Water supply is a matter of security. An Underground Cistern Vault supplied with water from a hundred mile long Aqueduct.

GARTEROBES: These are toilets for those who don’t want to squat over a bowl in their bedroom.

STAIRS & HALLS: These are designed to keep people from wandering though the private residences of others. Privacy is an issue. There might be private staircases and Halls to provide secrecy of movement and interaction.

FIREPLACES AND HEATING: There are likely a few permanent Fireplaces with Braziers (Bowls on Tripods where coals burn) to heat smaller Rooms, perhaps Cupboards lined with copper sheet to draw heat from a Chimney Pipe and heat the air through convection causing hot air to rise up to different floors.

USING THE SOCIAL RANK MAGNIFIER: Multiply the size of Rooms used by a Ruler by the Rank Modifier.

BEYOND CASTLES

'Nice City...'

These same rules can be used to calculate the size of churches in cities, the needs of residents and soforth.
Worse still...one structure might be retasked to another purpose. A church might be turned into a tavern. The old crypts the cellars. So where the castle chapel might have fitted fifty, the town temple might be big enough to take a thousand. The blackwater inn is a hall, a kitchen, cellars, and rooms for occupants and guests...it could be an old tower keep from when the town was a village.
A roadside shrine might be a church with no seating.