diff --git a/packages/domain/src/billing-period.test.ts b/packages/domain/src/billing-period.test.ts
new file mode 100644
index 0000000..53a3a8f
--- /dev/null
+++ b/packages/domain/src/billing-period.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, test } from 'bun:test'
+
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+import { BillingPeriod } from './billing-period'
+
+describe('BillingPeriod', () => {
+  test('parses canonical YYYY-MM format', () => {
+    const period = BillingPeriod.fromString('2026-03')
+
+    expect(period.year).toBe(2026)
+    expect(period.month).toBe(3)
+    expect(period.toString()).toBe('2026-03')
+  })
+
+  test('rejects malformed format', () => {
+    expect(() => BillingPeriod.fromString('2026/03')).toThrow(
+      new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_BILLING_PERIOD,
+        'Billing period must match YYYY-MM: 2026/03'
+      )
+    )
+  })
+
+  test('navigates next and previous correctly', () => {
+    const december = BillingPeriod.from(2026, 12)
+
+    expect(december.next().toString()).toBe('2027-01')
+    expect(december.previous().toString()).toBe('2026-11')
+  })
+
+  test('compares periods', () => {
+    const left = BillingPeriod.from(2026, 3)
+    const right = BillingPeriod.from(2026, 4)
+
+    expect(left.compare(right)).toBe(-1)
+    expect(right.compare(left)).toBe(1)
+    expect(left.compare(BillingPeriod.from(2026, 3))).toBe(0)
+  })
+})
diff --git a/packages/domain/src/billing-period.ts b/packages/domain/src/billing-period.ts
new file mode 100644
index 0000000..4ac5942
--- /dev/null
+++ b/packages/domain/src/billing-period.ts
@@ -0,0 +1,98 @@
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+
+const BILLING_PERIOD_PATTERN = /^(\d{4})-(\d{2})$/
+
+function isIntegerInRange(value: number, min: number, max: number): boolean {
+  return Number.isInteger(value) && value >= min && value <= max
+}
+
+export class BillingPeriod {
+  readonly year: number
+  readonly month: number
+
+  private constructor(year: number, month: number) {
+    this.year = year
+    this.month = month
+  }
+
+  static from(year: number, month: number): BillingPeriod {
+    if (!isIntegerInRange(year, 1970, 9999)) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_BILLING_PERIOD,
+        `Invalid billing year: ${year}`
+      )
+    }
+
+    if (!isIntegerInRange(month, 1, 12)) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_BILLING_PERIOD,
+        `Invalid billing month: ${month}`
+      )
+    }
+
+    return new BillingPeriod(year, month)
+  }
+
+  static fromString(value: string): BillingPeriod {
+    const match = BILLING_PERIOD_PATTERN.exec(value)
+
+    if (!match) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_BILLING_PERIOD,
+        `Billing period must match YYYY-MM: ${value}`
+      )
+    }
+
+    const [, yearString, monthString] = match
+
+    return BillingPeriod.from(Number(yearString), Number(monthString))
+  }
+
+  static fromDate(date: Date): BillingPeriod {
+    return BillingPeriod.from(date.getUTCFullYear(), date.getUTCMonth() + 1)
+  }
+
+  next(): BillingPeriod {
+    if (this.month === 12) {
+      return BillingPeriod.from(this.year + 1, 1)
+    }
+
+    return BillingPeriod.from(this.year, this.month + 1)
+  }
+
+  previous(): BillingPeriod {
+    if (this.month === 1) {
+      return BillingPeriod.from(this.year - 1, 12)
+    }
+
+    return BillingPeriod.from(this.year, this.month - 1)
+  }
+
+  compare(other: BillingPeriod): -1 | 0 | 1 {
+    if (this.year < other.year) {
+      return -1
+    }
+
+    if (this.year > other.year) {
+      return 1
+    }
+
+    if (this.month < other.month) {
+      return -1
+    }
+
+    if (this.month > other.month) {
+      return 1
+    }
+
+    return 0
+  }
+
+  equals(other: BillingPeriod): boolean {
+    return this.year === other.year && this.month === other.month
+  }
+
+  toString(): string {
+    return `${this.year.toString().padStart(4, '0')}-${this.month.toString().padStart(2, '0')}`
+  }
+}
diff --git a/packages/domain/src/errors.ts b/packages/domain/src/errors.ts
new file mode 100644
index 0000000..716c202
--- /dev/null
+++ b/packages/domain/src/errors.ts
@@ -0,0 +1,19 @@
+export class DomainError extends Error {
+  readonly code: string
+
+  constructor(code: string, message: string) {
+    super(message)
+    this.name = 'DomainError'
+    this.code = code
+  }
+}
+
+export const DOMAIN_ERROR_CODE = {
+  INVALID_MONEY_AMOUNT: 'INVALID_MONEY_AMOUNT',
+  INVALID_MONEY_MAJOR_FORMAT: 'INVALID_MONEY_MAJOR_FORMAT',
+  CURRENCY_MISMATCH: 'CURRENCY_MISMATCH',
+  INVALID_SPLIT_PARTS: 'INVALID_SPLIT_PARTS',
+  INVALID_SPLIT_WEIGHTS: 'INVALID_SPLIT_WEIGHTS',
+  INVALID_BILLING_PERIOD: 'INVALID_BILLING_PERIOD',
+  INVALID_ENTITY_ID: 'INVALID_ENTITY_ID'
+} as const
diff --git a/packages/domain/src/ids.test.ts b/packages/domain/src/ids.test.ts
new file mode 100644
index 0000000..3d02540
--- /dev/null
+++ b/packages/domain/src/ids.test.ts
@@ -0,0 +1,34 @@
+import { describe, expect, test } from 'bun:test'
+
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+import { BillingCycleId, HouseholdId, MemberId, PurchaseEntryId } from './ids'
+
+describe('IDs', () => {
+  test('creates and compares typed ids', () => {
+    const left = MemberId.from('member_1')
+    const right = MemberId.from('member_1')
+
+    expect(left.equals(right)).toBe(true)
+    expect(left.toString()).toBe('member_1')
+  })
+
+  test('typed ids with same value and different type are not equal', () => {
+    const member = MemberId.from('abc')
+    const household = HouseholdId.from('abc')
+
+    expect(member.equals(household)).toBe(false)
+  })
+
+  test('rejects invalid id values', () => {
+    expect(() => BillingCycleId.from('')).toThrow(
+      new DomainError(DOMAIN_ERROR_CODE.INVALID_ENTITY_ID, 'BillingCycleId cannot be empty')
+    )
+
+    expect(() => PurchaseEntryId.from('bad value with space')).toThrow(
+      new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_ENTITY_ID,
+        'PurchaseEntryId contains invalid characters: bad value with space'
+      )
+    )
+  })
+})
diff --git a/packages/domain/src/ids.ts b/packages/domain/src/ids.ts
new file mode 100644
index 0000000..4249823
--- /dev/null
+++ b/packages/domain/src/ids.ts
@@ -0,0 +1,60 @@
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+
+const ID_PATTERN = /^[a-zA-Z0-9][a-zA-Z0-9:_-]{0,127}$/
+
+function normalizeId(raw: string, label: string): string {
+  const value = raw.trim()
+
+  if (!value) {
+    throw new DomainError(DOMAIN_ERROR_CODE.INVALID_ENTITY_ID, `${label} cannot be empty`)
+  }
+
+  if (!ID_PATTERN.test(value)) {
+    throw new DomainError(
+      DOMAIN_ERROR_CODE.INVALID_ENTITY_ID,
+      `${label} contains invalid characters: ${value}`
+    )
+  }
+
+  return value
+}
+
+abstract class BaseId {
+  readonly value: string
+
+  protected constructor(value: string) {
+    this.value = value
+  }
+
+  equals(other: BaseId): boolean {
+    return this.value === other.value && this.constructor === other.constructor
+  }
+
+  toString(): string {
+    return this.value
+  }
+}
+
+export class HouseholdId extends BaseId {
+  static from(value: string): HouseholdId {
+    return new HouseholdId(normalizeId(value, 'HouseholdId'))
+  }
+}
+
+export class MemberId extends BaseId {
+  static from(value: string): MemberId {
+    return new MemberId(normalizeId(value, 'MemberId'))
+  }
+}
+
+export class BillingCycleId extends BaseId {
+  static from(value: string): BillingCycleId {
+    return new BillingCycleId(normalizeId(value, 'BillingCycleId'))
+  }
+}
+
+export class PurchaseEntryId extends BaseId {
+  static from(value: string): PurchaseEntryId {
+    return new PurchaseEntryId(normalizeId(value, 'PurchaseEntryId'))
+  }
+}
diff --git a/packages/domain/src/index.ts b/packages/domain/src/index.ts
index 1196bf9..9ba467e 100644
--- a/packages/domain/src/index.ts
+++ b/packages/domain/src/index.ts
@@ -1 +1,13 @@
-export const domainReady = true
+export { BillingPeriod } from './billing-period'
+export { DOMAIN_ERROR_CODE, DomainError } from './errors'
+export { BillingCycleId, HouseholdId, MemberId, PurchaseEntryId } from './ids'
+export { CURRENCIES, Money } from './money'
+export type { CurrencyCode } from './money'
+export type {
+  SettlementInput,
+  SettlementMemberInput,
+  SettlementMemberLine,
+  SettlementPurchaseInput,
+  SettlementResult,
+  UtilitySplitMode
+} from './settlement-primitives'
diff --git a/packages/domain/src/money.test.ts b/packages/domain/src/money.test.ts
new file mode 100644
index 0000000..1257c1f
--- /dev/null
+++ b/packages/domain/src/money.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, test } from 'bun:test'
+
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+import { Money } from './money'
+
+describe('Money', () => {
+  test('parses major units and formats back deterministically', () => {
+    const money = Money.fromMajor('12.34', 'GEL')
+
+    expect(money.amountMinor).toBe(1234n)
+    expect(money.toMajorString()).toBe('12.34')
+  })
+
+  test('rejects non-integer minor units', () => {
+    expect(() => Money.fromMinor(10.5, 'GEL')).toThrow(
+      new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_MONEY_AMOUNT,
+        'Money minor amount must be an integer'
+      )
+    )
+  })
+
+  test('adds and subtracts money in same currency', () => {
+    const base = Money.fromMinor(1000n, 'USD')
+    const delta = Money.fromMinor(250n, 'USD')
+
+    expect(base.add(delta).amountMinor).toBe(1250n)
+    expect(base.subtract(delta).amountMinor).toBe(750n)
+  })
+
+  test('throws on currency mismatch', () => {
+    const gel = Money.fromMinor(1000n, 'GEL')
+    const usd = Money.fromMinor(1000n, 'USD')
+
+    expect(() => gel.add(usd)).toThrow(
+      new DomainError(
+        DOMAIN_ERROR_CODE.CURRENCY_MISMATCH,
+        'Money operation currency mismatch: GEL vs USD'
+      )
+    )
+  })
+
+  test('splits evenly with deterministic remainder allocation', () => {
+    const amount = Money.fromMinor(10n, 'GEL')
+    const parts = amount.splitEvenly(3)
+
+    expect(parts.map((part) => part.amountMinor)).toEqual([4n, 3n, 3n])
+    expect(parts.reduce((sum, current) => sum + current.amountMinor, 0n)).toBe(10n)
+  })
+
+  test('splits by weights deterministically', () => {
+    const amount = Money.fromMinor(100n, 'GEL')
+    const parts = amount.splitByWeights([3, 2, 1])
+
+    expect(parts.map((part) => part.amountMinor)).toEqual([50n, 33n, 17n])
+    expect(parts.reduce((sum, current) => sum + current.amountMinor, 0n)).toBe(100n)
+  })
+
+  test('splits negative values with same deterministic rules', () => {
+    const amount = Money.fromMinor(-10n, 'GEL')
+    const parts = amount.splitEvenly(3)
+
+    expect(parts.map((part) => part.amountMinor)).toEqual([-4n, -3n, -3n])
+    expect(parts.reduce((sum, current) => sum + current.amountMinor, 0n)).toBe(-10n)
+  })
+})
diff --git a/packages/domain/src/money.ts b/packages/domain/src/money.ts
new file mode 100644
index 0000000..f8b613c
--- /dev/null
+++ b/packages/domain/src/money.ts
@@ -0,0 +1,259 @@
+import { DOMAIN_ERROR_CODE, DomainError } from './errors'
+
+export const CURRENCIES = ['GEL', 'USD'] as const
+
+export type CurrencyCode = (typeof CURRENCIES)[number]
+
+const MAJOR_MONEY_PATTERN = /^([+-]?)(\d+)(?:\.(\d{1,2}))?$/
+
+function isIntegerNumber(value: number): boolean {
+  return Number.isFinite(value) && Number.isInteger(value)
+}
+
+function parseMinorUnits(value: bigint | number | string): bigint {
+  if (typeof value === 'bigint') {
+    return value
+  }
+
+  if (typeof value === 'number') {
+    if (!isIntegerNumber(value)) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_MONEY_AMOUNT,
+        'Money minor amount must be an integer'
+      )
+    }
+
+    return BigInt(value)
+  }
+
+  if (!/^[+-]?\d+$/.test(value)) {
+    throw new DomainError(
+      DOMAIN_ERROR_CODE.INVALID_MONEY_AMOUNT,
+      'Money minor amount string must contain only integer digits'
+    )
+  }
+
+  return BigInt(value)
+}
+
+function parseMajorUnits(value: string): bigint {
+  const trimmed = value.trim()
+  const match = MAJOR_MONEY_PATTERN.exec(trimmed)
+
+  if (!match) {
+    throw new DomainError(
+      DOMAIN_ERROR_CODE.INVALID_MONEY_MAJOR_FORMAT,
+      `Invalid money major format: ${value}`
+    )
+  }
+
+  const [, sign, wholePart, fractionalPart = ''] = match
+  const normalizedFraction = fractionalPart.padEnd(2, '0')
+  const composed = `${wholePart}${normalizedFraction}`
+  const signPrefix = sign === '-' ? '-' : ''
+
+  return BigInt(`${signPrefix}${composed}`)
+}
+
+function ensureSupportedCurrency(currency: string): CurrencyCode {
+  if ((CURRENCIES as readonly string[]).includes(currency)) {
+    return currency as CurrencyCode
+  }
+
+  throw new DomainError(DOMAIN_ERROR_CODE.INVALID_MONEY_AMOUNT, `Unsupported currency: ${currency}`)
+}
+
+function formatMajorUnits(minor: bigint): string {
+  const sign = minor < 0n ? '-' : ''
+  const absolute = minor < 0n ? -minor : minor
+  const whole = absolute / 100n
+  const fraction = absolute % 100n
+  const fractionString = fraction.toString().padStart(2, '0')
+
+  return `${sign}${whole.toString()}.${fractionString}`
+}
+
+export class Money {
+  readonly amountMinor: bigint
+  readonly currency: CurrencyCode
+
+  private constructor(amountMinor: bigint, currency: CurrencyCode) {
+    this.amountMinor = amountMinor
+    this.currency = currency
+  }
+
+  static fromMinor(amountMinor: bigint | number | string, currency: CurrencyCode = 'GEL'): Money {
+    const supportedCurrency = ensureSupportedCurrency(currency)
+
+    return new Money(parseMinorUnits(amountMinor), supportedCurrency)
+  }
+
+  static fromMajor(amountMajor: string, currency: CurrencyCode = 'GEL'): Money {
+    const supportedCurrency = ensureSupportedCurrency(currency)
+
+    return new Money(parseMajorUnits(amountMajor), supportedCurrency)
+  }
+
+  static zero(currency: CurrencyCode = 'GEL'): Money {
+    return Money.fromMinor(0n, currency)
+  }
+
+  add(other: Money): Money {
+    this.assertSameCurrency(other)
+
+    return new Money(this.amountMinor + other.amountMinor, this.currency)
+  }
+
+  subtract(other: Money): Money {
+    this.assertSameCurrency(other)
+
+    return new Money(this.amountMinor - other.amountMinor, this.currency)
+  }
+
+  multiplyBy(multiplier: bigint | number): Money {
+    const parsedMultiplier = typeof multiplier === 'number' ? BigInt(multiplier) : multiplier
+
+    if (typeof multiplier === 'number' && !isIntegerNumber(multiplier)) {
+      throw new DomainError(DOMAIN_ERROR_CODE.INVALID_MONEY_AMOUNT, 'Multiplier must be an integer')
+    }
+
+    return new Money(this.amountMinor * parsedMultiplier, this.currency)
+  }
+
+  splitEvenly(parts: number): readonly Money[] {
+    if (!isIntegerNumber(parts) || parts <= 0) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_SPLIT_PARTS,
+        'Split parts must be a positive integer'
+      )
+    }
+
+    return this.splitByWeights(Array.from({ length: parts }, () => 1n))
+  }
+
+  splitByWeights(weightsInput: readonly (bigint | number)[]): readonly Money[] {
+    if (weightsInput.length === 0) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_SPLIT_WEIGHTS,
+        'At least one weight is required'
+      )
+    }
+
+    const weights = weightsInput.map((weight) => {
+      const parsed = typeof weight === 'number' ? BigInt(weight) : weight
+
+      if (typeof weight === 'number' && !isIntegerNumber(weight)) {
+        throw new DomainError(
+          DOMAIN_ERROR_CODE.INVALID_SPLIT_WEIGHTS,
+          'Split weights must be integers'
+        )
+      }
+
+      if (parsed <= 0n) {
+        throw new DomainError(
+          DOMAIN_ERROR_CODE.INVALID_SPLIT_WEIGHTS,
+          'Split weights must be positive'
+        )
+      }
+
+      return parsed
+    })
+
+    const totalWeight = weights.reduce((sum, current) => sum + current, 0n)
+
+    if (totalWeight <= 0n) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.INVALID_SPLIT_WEIGHTS,
+        'Total split weight must be positive'
+      )
+    }
+
+    const isNegative = this.amountMinor < 0n
+    const absoluteAmount = isNegative ? -this.amountMinor : this.amountMinor
+
+    const baseAllocations = weights.map((weight) => (absoluteAmount * weight) / totalWeight)
+    const remainders = weights.map((weight, index) => ({
+      index,
+      remainder: (absoluteAmount * weight) % totalWeight
+    }))
+
+    const allocatedBase = baseAllocations.reduce((sum, current) => sum + current, 0n)
+    const leftover = absoluteAmount - allocatedBase
+
+    remainders.sort((left, right) => {
+      if (left.remainder === right.remainder) {
+        return left.index - right.index
+      }
+
+      return left.remainder > right.remainder ? -1 : 1
+    })
+
+    const finalAllocations = [...baseAllocations]
+    for (let offset = 0n; offset < leftover; offset += 1n) {
+      const target = remainders[Number(offset)]
+      if (!target) {
+        break
+      }
+
+      const currentAllocation = finalAllocations[target.index]
+      if (currentAllocation === undefined) {
+        throw new DomainError(
+          DOMAIN_ERROR_CODE.INVALID_SPLIT_WEIGHTS,
+          'Unexpected split allocation index state'
+        )
+      }
+
+      finalAllocations[target.index] = currentAllocation + 1n
+    }
+
+    return finalAllocations.map(
+      (allocatedMinor) => new Money(isNegative ? -allocatedMinor : allocatedMinor, this.currency)
+    )
+  }
+
+  equals(other: Money): boolean {
+    return this.currency === other.currency && this.amountMinor === other.amountMinor
+  }
+
+  isNegative(): boolean {
+    return this.amountMinor < 0n
+  }
+
+  isZero(): boolean {
+    return this.amountMinor === 0n
+  }
+
+  compare(other: Money): -1 | 0 | 1 {
+    this.assertSameCurrency(other)
+
+    if (this.amountMinor < other.amountMinor) {
+      return -1
+    }
+
+    if (this.amountMinor > other.amountMinor) {
+      return 1
+    }
+
+    return 0
+  }
+
+  toMajorString(): string {
+    return formatMajorUnits(this.amountMinor)
+  }
+
+  toJSON(): { amountMinor: string; currency: CurrencyCode } {
+    return {
+      amountMinor: this.amountMinor.toString(),
+      currency: this.currency
+    }
+  }
+
+  private assertSameCurrency(other: Money): void {
+    if (this.currency !== other.currency) {
+      throw new DomainError(
+        DOMAIN_ERROR_CODE.CURRENCY_MISMATCH,
+        `Money operation currency mismatch: ${this.currency} vs ${other.currency}`
+      )
+    }
+  }
+}
diff --git a/packages/domain/src/settlement-primitives.ts b/packages/domain/src/settlement-primitives.ts
new file mode 100644
index 0000000..c09d56f
--- /dev/null
+++ b/packages/domain/src/settlement-primitives.ts
@@ -0,0 +1,44 @@
+import type { BillingPeriod } from './billing-period'
+import type { BillingCycleId, MemberId, PurchaseEntryId } from './ids'
+import type { Money } from './money'
+
+export type UtilitySplitMode = 'equal' | 'weighted_by_days'
+
+export interface SettlementMemberInput {
+  memberId: MemberId
+  active: boolean
+  utilityDays?: number
+}
+
+export interface SettlementPurchaseInput {
+  purchaseId: PurchaseEntryId
+  payerId: MemberId
+  amount: Money
+  description?: string
+}
+
+export interface SettlementInput {
+  cycleId: BillingCycleId
+  period: BillingPeriod
+  rent: Money
+  utilities: Money
+  utilitySplitMode: UtilitySplitMode
+  members: readonly SettlementMemberInput[]
+  purchases: readonly SettlementPurchaseInput[]
+}
+
+export interface SettlementMemberLine {
+  memberId: MemberId
+  rentShare: Money
+  utilityShare: Money
+  purchaseOffset: Money
+  netDue: Money
+  explanations: readonly string[]
+}
+
+export interface SettlementResult {
+  cycleId: BillingCycleId
+  period: BillingPeriod
+  lines: readonly SettlementMemberLine[]
+  totalDue: Money
+}