Pemberley, Mr. Darcy's seat. Curious, however, in that while by modern standards it's quite flashy, by the standards of the time it's quite a restrained and tasteful property, which is one of the things that indicates to Elizabeth Bennet that Mr. Darcy's Hidden Depths reveal him to be a more modest, humble and decent man than first impressions indicate.
Rosings Park, the home of his aunt Lady Catherine de Bourgh; in keeping with his aunt's overall foolishness, snobbery and lack of decorum, it's a lot more gaudy and show-offy.
Sir Thomas and Lady Bertram's titular mansion. It's quite grand with extensive property. Mrs. Norris still contrives to make Fanny's rooms in the attic cold and uncomfortable, though, until Sir Thomas finds out about it.
Mr. Rushworth is fabulously wealthy and his opulent family seat is called Sotherton. It has a huge house and large gardens and parks.
Donwell Abbey is Mr. Knightley's family estate. It's very old, traditional, with gardens and orchards, a lime avenue, farms and tenants renting the land. The house itself is huge and some of the rooms are furnished exquisitely.
Hartfield, which belongs to the Woodhouses. It's smaller than Donwell Abbey - just a "notch" in the estate - but it has a nice park, a farm attached, and it's modern and elegant, decorated and furnished with Emma's taste.
Northanger Abbey: The title location is a well-furnished, comfortable, with nicely updated decor despite the age of the building... this is rather disappointing to Catherine, who was expecting a haunted house to fulfill her Gothic inclinations.
Sense and Sensibility opens at the Dashwood estate, Norland Park, which is a beautiful mansion in Devonshire; the late Mr. Dashwood's widow and daughters all love it and are sorry to leave it, especially since their stepson/half-brother's wife Fanny will impose her tastes upon it and do things like pull down the old trees to make room for a greenhouse.
Mrs. Dashwood and her daughters then remove to Barton Cottage, which is part of the property of Barton Park, home of Mrs. Dashwood's cousin; it's a massive estate with fields and woods, and Barton Park is a large and elegantly furnished manor.
Elinor and Marianne later accompany Mrs. Jennings to her unnamed house in London, which is also lavish and spacious. Later still, they visit the Palmers at their estate, Cleveland, which is yet another sprawling property with a Grecian temple on the grounds.
In the Foundation novels, we see the Emperor of the Galaxy lives on a 100 acre palace on the capital world of Trantor. Noteworthy since the rest of the planet is completely covered in a series of metal domes.
Manderley, in Daphne du Maurier's Rebecca, is the Cornish country estate of the wealthy Englishman Maximilian de Winter. It features heirlooms, a full staff, and is open to the public on certain days.
The Grosvenor Square mansion of the outrageously wealthy financier Augustus Melmotte in The Way We Live Now.
In another one of his book series Blandings Castle, the titular house is, as the title hints at, very large and home to a wide selection of characters.
Hercule Poirot frequently provides his services as a detective to upper-class residents of big, fancy houses. The Poirot TV adaptation even has Poirot and his friends reside in a flat at Whitehaven Mansions.
The Secret of Chimneys is largely set in the big house that gives the novel its title.
Thornfield Hall, the Gothic estate of the wealthy Edward Rochester in Jane Eyre.
The increasingly decrepit Hundreds Hall in Sarah Waters' The Little Stranger.
When an Ugly becomes a Pretty in Uglies, they get moved from a dorm to a Big Fancy House.
Homeward in J.P Martin's Uncle series. It's so big that the owner hasn't met a tenth of the people who also live there. It has a railway station that he didn't know about until the second book, and the most pimped out library possible, among countless other things.
Fowl Manor in the outskirts of Dublin, Ireland from Artemis Fowl. It's 200 freaking acres.
Baskerville Hall is probably the most well-known example in the Sherlock Holmes canon, but there are several instances of him visiting the sprawling country homes of the rich and powerful (and, occasionally, criminal).
In Buddenbrooks, Thomas builds one, but later feels exhausted and regrets building such an expensive home. Even the house the family moves in later (after their downfall has become obvious) would probably qualify.
Subverted in Malevil. The titular Malevil is a large English castle from The Hundred Years War, sitting on a cliff with accompanying grounds. Emmanuel is not a wealthy man, upper-middle class at best, and nor was his uncle who left the inheritance he buys Malevil with. The property was sold "cheap" being considered a bad investment; the castle officially condemned and the grounds too unkempt to be worth the expense and hassle of restoration or clearing.
In The Good Earth, the rich family on the outskirts of the protagonist's home town and the rich family in the city both have this. The one in the city is so big that an entire tent city is spring up leaning on the wall around the estate.
Notably in the Aunt Dimity series, Penford Hall, seat of the Duke of Penford in Aunt Dimity and the Duke; Hailesham Park, the seat of the Earl of Elstyn and the setting for Aunt Dimity Takes a Holiday; and Dundrillin Castle, Sir Percy's Scottish island retreat in Aunt Dimity and the Deep Blue Sea.
In Animorphs, the group breaks into the mansion of Joe Bob Fenestre, a near- Captain Ersatz of Bill Gates, who has a ton of security measures in it. It gets burned down in the end.
In Honor Harrington, the eponymous heroine has acquired several through the course of the series. Harrington House on Grayson (which doubles as headquarters for the local government), her house on Manticore, her duchy on Gryphon, and her family's not-inconsiderable home on Sphinx.
Hoffmann's house in The Fear Index is truly massive and costs around sixty million dollars. As Hoffmann is a recluse it's completely unnecessary but he bought it because that's what you do when you're rich.
Foxworth Hall in Flowers in the Attic is so big that when the grandmother arranges to have the children locked away, she can lock up a whole wing just to make sure no one hears them.
Kyle in the Mercy Thompson books has one. While a Big Fancy House isn't a surprise when you're a successful divorce lawyer, Kyle's home also doubles as a shelter for his clients in cases involving Domestic Abuse, with all the creature comforts needed to distract the kids and state-of-the-art security systems in case angry husbands show up.
Belle Rive in The Sookie Stackhouse Mysteries is the most desirable address in Bon Temps. It's mentioned that there are many women in the town who would marry Andy Bellefleur just to be able to live there. His eventual wife Halleigh wants much more modest accommodations, though.
The Bosses' mansions in Clocks that Don't Tick, which are made even more impressive by the fact they're built inside mountains. The one shown was said to include everything from the mercenary's quarters to swimming pools to indoor gold courses. The main level where the Bosses reside is made to resemble a luxury hotel with a courtyard. Said courtyard includes multiple hot tubs and golden fountains portraying Hope, her dress embedded with dozens of jewels. Above it all is a ceiling perfectly made to look like a starry sky. The mansion is also the only place shown that features futuristic technology appropriate for the novel's setting of five hundred years in the future. For example, there are holographic control panels that can be summoned by the Bosses making a certain gesture.
There are a fair few in the Village Tales series, although they're all in the best of taste and rather restrained, really, dontcherknow. The Duke of Taunton, naturally, has several: Wolfdown House outside Wolfdown Abbas being his primary seat. His father wisely bought in the rectories and vicarages, to preserve them, during the Great C of E Fire Sale, so the Rector is as comfortably housed as is Sir Thomas Douty at Davill Court or the Salmons at Charltons or Teddy and Edmond at Chalkhills. The Mirzas and the patriarch of that family, HH the Nawab, have, like the Duke and all his titled relations, several all over the UK. And the Duke manages to claw back old Lord Mallerstang's ancestral hall for him – from the National Trust – when it becomes clear the Duke's nephew will be inheriting that title as well as the dukedom and Mallerstang wishes to leave the lad something.
My Sweet Audrina: Whitefern used to be this back when the family of the same name still had power, but by the time the story takes place it has been falling apart for years. Damian begins to have it repaired later in the book after getting more money from less-than-legal stockbroking practices.
Farthinggale Manor in the Casteel series. Tony proudly proclaims that each generation of the Tatterton family makes an effort to improve it. However when it is revisited in Gates of Paradise, it has fallen into ruin due to Tony's declining sanity.